The best example of one-to-many communication among the options provided is:
A brand’s Facebook page.
This is because a brand’s Facebook page allows the brand to post updates, news, and advertisements that can be seen by many followers or users at once, while individual users can comment or react but do not communicate back in a direct one-on-one manner like in a chat room.
While the other options involve communication from one source to many, the brand’s Facebook page is a more direct representation of ongoing communication with an audience in a social media context.