In the Microsoft Bot Framework SDK, bots communicate using activities (messages, events, conversation updates, etc.).
To respond to events (like conversation started, member added, or custom event activities), you use an ActivityHandler.
The ActivityHandler class provides methods you can override, such as OnMessageActivityAsync , OnMembersAddedAsync , or custom event handlers, to send custom text responses.
Other options:
Adaptive card (A) # used for rich UI responses, not for handling events.
Dialog (C) # used for managing conversation flow, not event handling.
Skill (D) # used to integrate another bot's functionality, not for handling custom events.
Reference: ActivityHandler class in Bot Framework