Which contraction correctly fills the blank in the sentence: "_____ coming to dinner tonight?"

The correct contraction to fill in the blank is "who's," which is a contraction of "who is." This contraction is appropriate in the context of the sentence, which is asking if someone (referring to a person) is coming to dinner tonight.

Using "who" by itself does not function grammatically, as it does not convey the state of being that is required in the sentence. The contraction effectively combines the pronoun "who" with the verb "is," making it clear that the sentence is inquiring about the identity of the person coming to dinner.

It's also important to note that "what's" (short for "what is") could lead to confusion, as it does not appropriately relate to the subject of the inquiry in this context. The term "whats" lacks the apostrophe that indicates it is a contraction and therefore is not grammatically correct in standard English usage. Hence, "who's" fits perfectly to complete the sentence correctly.