Which sentence is grammatically correct?

The sentence "Which one do you want?" is grammatically correct because it uses the appropriate form of the interrogative pronoun "which" to ask about a specific item among options. The structure follows standard English grammar rules for questions, including proper subject-verb agreement and clarity.

In this case, "which" is correctly used to refer to a choice or selection, and "one" acts as a noun that indicates the item in question. The use of "do you want" is a proper construction in direct questions, maintaining the correct form of the verb.

The other options contain grammatical errors. For instance, the use of "won" instead of "one" in a separate choice, where "won" denotes a victory and doesn't fit in the context of asking about a selection. In another choice, "whose" is used incorrectly in the context of ownership rather than selection, which alters the intended meaning of the sentence. Lastly, "who's" is a contraction for "who is," which also does not suit the context of selecting an item, leading to confusion in the sentence.

Thus, the clarity and correctness of "Which one do you want?" make it the only appropriate choice.