A. $
B. #
C. @
D. none
On Twitter, the "@" symbol is used to denote a user's handle. The handle is a unique identifier for a Twitter account and is used in the format "@username." For example, if a user's handle is "exampleuser," their Twitter handle would be "@exampleuser." The "@" symbol is an essential part of mentioning or tagging users in tweets, allowing users to reference and communicate with each other on the platform.