The app builder would enable this ability in Chatter Settings. Chatter Settings is a setup menu that allows an app builder to configure various aspects of Chatter functionality, such as feeds, groups, email notifications, and more. One of the options in Chatter Settings is Enable Approvals via Chatter, which allows users to approve or reject requests from their team via Chatter posts or comments. Option A, B, and D are not places where an app builder would enable this ability.