GMAT Verbal Section | One of the X who/that - Sentence Correction/ Sub-verb agreement

I am bit confused on the subject verb agreement on the below one particularly.

This is one of the cars that belong/belongs to him.

This is one of the questions that is/are incorrect.

Can someone explain this ???

If I go by the logic of quantity preposition phrases, both one of the cars and one of the question will be singular.

Thanks!!