Is it "which of these is...", or "which of these are..."

I came accross a question in my maths homework which was worded as follows (emphasis mine).

Which of the following claims is correct?

After which it goes on to list a collection of claims, two of which are true. My question is if it is grammatically correct to say "is correct" in this context, or if the correct wording is "are correct", assuming the question doesn’t want to restrict the set of correct answers to just one.

I think "are correct" is acceptable because the first part, "which of the following" is referring to a subset of the answers (of unspecified size), so it should be referred to in plural.

However, it seems to me that "is correct" can also be acceptable because "which of the following" might be referring to each one individually, then asking about each claim if it is correct.

So does saying "is correct" limit the solution set to a size of one or not?

Edit: I know the lecturer (who wrote the question) meant there to be multiple correct answers, I am interested in the grammatical correctness of the sentence, and its reasoning.