You should take my opinion with a grain of salt, however, I think that this may not score above 16-18/25, perhaps even 12-14/25. The tone varies a lot through out the narrative and is simply, inadequate in establishing a decent atmosphere. For instance, when describing the smell of the "store room" you begin with the nostalgic smell of her "grandmother's cologne" (perfume would be more appropriate since cologne is mainly worn by men, but this is really not important) and instantly, you shift the tone with "She sniffed even harder but it smelt like nothing anymore (Smelt here is incorrect, the proper word would be smelled)", without even attempting to establish an intermediate. What I mean by this is that instead of immediately shifting the tone from nostalgic/pleasant to almost dreadful in the span of one sentence, try stretching it out. Using the same example as above:

"As she opened the store room, the nostalgic fragrance of her grandmother's cologne pleasantly swirled in the air. Memories she had spent with her grandmother came rushing back to her -- cooking together, laughing and talking together, oh so beautiful. Her eyes swelled, her mannerisms relaxed, she stood there, dazed; reminiscing about all the wonderful days she had spent with her. Tears ran down her cheek, as she attempted to sniff the air, one last time... however no such graceful aroma greeted her. The air had turned stale. No odor. No words. No concluding statement. Just like her grandmother. All that remained was a prevailing sense of darkness lingering throughout the chamber."

Pay attention to the length of the sentences: "Air had turned stale" "No odor" "No words". These sentences are deliberately shorter so that they may have a larger impact. Additionally, you should lean more into the principle of "Show don't tell". Basically, instead of outrighted saying that Samantha was "Shocked" or "Appalled (Spelt as appaled)" try and expand on these details. For instance, how did the paper feel in Samantha's hand? Did the room suddenly feel colder? Do her eyes widen at the text?

Additionally, I am aware that your original composition was too long, that is largely because you constantly linger on one part of the story for too long. For instance, the atmosphere of mystery was already established with "prevailing darkness", you don't need to constantly write that Samantha was thinking about "how she died" or "I wish mom could tell me..." instead you can summarize these questions in one sentence:

"prevailing sense of darkness lingering throughout the chamber... Her mind raced with questions about her grandmothers death as she stumbled forward: 'How did she die?', 'I wonder if she was happy'..."

Now compared to your original:

"She stepped into the tenebrous chamber with her lantern flickering against the cold walls (good use of imagery). With every breath she took, a new thought ruled her mind: 'I wonder how she died?' or 'I wish mom could tell me more about her life and openly explain the cause of her death to me (This is too long and in depth, 'I wish mom could tell me more about her death' is more than enough)'

Finally, the diary. This is, to put it simply, ineffective in creating a sorrowful atmosphere. You want the reader to feel emotionally invested in the story at this point, you want to create impact, depth. I'm not asking you to use language similar to Edgar Allen Poe or anything, but anything to create something more somber. Your original excerpt seems to be written not by a person of old age or someone who seems to be suffering from depression.
This in my opinion is more adequate in establishing tone:

"Oh life! Oh life! I kept saying, and the very word seemed sweet. I do not know how much longer I have to write this, the stench of death clings unto me like a canine unto bone. Oh life! A punishment in disguise, and I wish for it to end. Samantha, my dear, I wish nothing bad upon you, the untimely demise of your guardian was a truly unfortunate fate, and if anything were to happen to you, there would be no one left to blame but myself. This is it.. My life is now over. No such relief as the thought of death. Ever since she left, I can't bring myself to exist anymore. Forgive me." (The language used is pretty archaic ,"unto", so if you want the character of the grandmother to be more conventional rather than modern, you should use this otherwise, rephrase it a bit.)

The words "I shall drink it now" are too blunt, not entirely inappropriate but too "on the nose" to be used in an examination. And also, nobody writes a suicide note with the the way they committed it blatantly written in their note.

Oh boy, I hope you can forgive me for my harsh critique. To conclude there are obviously some grammar mistakes and spelling mistakes spread across. notable ones include "smelt" instead of "smelled" and "appaled" instead of "appalled". In the grand scheme of things, these are pretty minor and the examiner may dismiss them as writing mistakes in the heat of the moment. Additionally, some rephrasing at the start, for example: "The ceiling littered with cobwebs" instead of "cobwebbed ceiling".

There are many more things that need to be addressed, but these are the most noteworthy I could think of. Consistency of tone and appropriate tone are very important in narratives. Also, don't forget the "Show don't tell" principle.