This question checks whether you know which pronoun form follows a linking verb like "are." Think of a linking verb as an equals sign joining the subject to the word after it, so both sides need to match in case.
Since sentence 1 is wrong and sentence 2 is fine, the statement that the first sentence has an error matches the facts, and the claim that the second sentence is erroneous does not, since sentence 2 is grammatically correct.
Let's summarize:
So only the first sentence, "Those are them," carries the error, which makes option A the correct choice. $\boxed{\text{Option A}}$