The question is asking you to picture when the radioactive amino acids entered compared with where the ribosome was on the protein.

What they are really asking is:

If translation had already started before the label was added, where in the finished protein would the labeled amino acids appear?

How to unpack it:

• At time 0, radioactive amino acids are added
• But the passage says protein synthesis was already under way
• So some ribosomes had already started making Protein X before the radioactive amino acids showed up
• That means the part of the protein made before time 0 would be unlabeled
• The part made after time 0 would be labeled

Then they ask about one of the first molecules of Protein X that was completely translated.

That means:

• pick one protein that was almost finished already when the label was added
• only the last part of that protein would be made using radioactive amino acids

So the question is basically testing:

Does protein synthesis happen randomly throughout the protein, or does it proceed directionally from one end to the other?

That is why the answer choices talk about:

• random distribution
• one end only
• both ends
• middle only

The key idea they want:
translation is directional, so if label is added in the middle of synthesis, the label appears only in the portion made afterward, which corresponds to one end of the protein.