Answer:
The answer is indeed correct as is.
Step-by-step explanation:
There is no reason to change the pronoun usage in the sentence "None of us wanted him and her to stop singing" for one of the options provided.
The pronouns we are supposed to correct do not function as subjects in this sentence, but as objects of the verb "wanted". Thus, changing any of them by using a subject pronoun - he or she - would make the sentence incorrect. Note that the options even mix both types of pronouns, subject and object - "he and her" and "him and she" -, which is an impossible usage, since both pronouns complete the same verb in the sentence.
The sentence is correct as is, using the object pronouns "him and her".