The correct pronoun to replace 'Quentin and Sandy' in the sentence is 'who', as it is the only option in the subjective case, which is required for subjects of sentences.
To choose the correct pronoun to replace the underlined noun in the sentence 'Danny is going to the performance tonight with Quentin and Sandy', we should look at the role that the noun plays in the sentence. In this case, Quentin and Sandy, along with Danny, are the subjects of the sentence as they are doing the action of going. Therefore, the correct pronoun should be in the subjective case because it is replacing a subject.
The subjective case pronouns that can function as subjects are: I, we, you, he/she/it, they, who/whoever. Given the options of 'her', 'whom', 'she', and 'who', the only pronoun in the subjective case is 'who'.
Pronouns like 'her' and 'she' can also be subjective pronouns but they refer to females, whereas 'Quentin and Sandy' is a mixed group, and 'whom' is an objective case pronoun used for objects of verbs or prepositions, which is not applicable in this context.
Therefore, the correct pronoun to replace 'Quentin and Sandy' in the given sentence is 'who'.