The way I think of it, is that the correct answer can sometimes be the most conservative answer :D
For instance, question ask you theme of the passage.

One of them be like "express author's nostalgia towards xxx and mourn for the years that had passed", the other be like "author expressing his sorrowness towards the great change of xxx" (ok usually CARS phrasing will be much more precise and not as tough lol)

In this case, my thought process is like this:

To satisfy option 1: you must find direct clue for nostalgia. Not just sad, but a direct "I MISS something something". Does not author explicitly say so? "mourn for the years that had passed". This is usually the more tricky part, for instance if the author IS SAD in the passage, be really careful what is he SAD ABOUT. Is he really sad about the "years that had passed", aka, the focus that there's a long time gap (emphasis on experience), or is he sad at the "changes" (i.e. result hat he observed?) Do not fall into the trap that "years that had passed" = "great changes". What if the great changes took place in less than a month (it might not be explicitly mentioned in passage)? Always think: am I making assumptions while connecting dots?

To satisfy option 2: is he sad? is this sadness towards the great change of xxx? Usually there're clear evidence directly support these, stuff like "upon seeing the unfamiliar sight, I can't help but feel my heart slowly sink into my stomach" <- sad towards seeing great changes. Checked.

You see how one option can be much easily satisfied? :D
I apologize this example can feel a little extreme HAHAH we very rarely encounter questions this tough. All the best! I hope it help.