Sentence correction asks for the repaired sentence, not the located error. Finding the fault is half the work.

Every option in a correction question is a whole sentence, and three of them repair the wrong thing or repair the right thing badly. The key is always the smallest change that makes the sentence correct.

Neither of the two files were traced. → Neither of the two files was traced.

the error is found and repaired minimally

Neither of the two files were traced. → The two files could not be traced.

a rewrite, not a correction

Where it bites

Options that fix the error and introduce a second one. Read the whole option, not just the part that changed.