Which word correctly fills in the blank: The dress ______ you wore at my birthday party is my favorite?

The word "that" is the correct choice to fill in the blank because it serves as a restrictive relative pronoun, introducing a defining clause that specifies which dress is being discussed. In this context, the clause "that you wore at my birthday party" provides essential information about the dress. By using "that," the sentence emphasizes the specific dress in question, making it clear and concise.

Using "that" indicates that the information following it is necessary for understanding which dress is a favorite. It helps to ensure the meaning is clear and direct, as it narrows down the subject to a particular item without adding unnecessary information. Thus, "The dress that you wore at my birthday party is my favorite" clearly identifies which dress is favored based on the occasion.