Which sentence is grammatically correct?

The choice "Who's coming to the party?" is grammatically correct because it uses "who's," which is a contraction for "who is." In this context, the sentence is asking about the subject of a future action (coming to the party) and is structured properly according to standard English grammar rules.

The other options contain errors related to the use of "whose" and "who's." "Whose" refers to possession, while "who's" is a contraction. Therefore, using "whose" in contexts where possession is not intended would be grammatically incorrect. Understanding the difference between these terms is crucial for identifying the correct usage in sentences.