Message shortcuts operate in the contextual action menu of an existing Slack message. Slack's official shortcut documentation specifies that message shortcuts appear in the context menus of non-ephemeral messages. Therefore, an ephemeral message cannot serve as the source message for a message shortcut, making B correct. This restriction follows from the underlying interaction model. Message shortcuts preserve context from the source message so an application can perform an action based on that message—for example, creating a ticket, escalating an incident, or sending content to an external system. Ephemeral messages have restricted visibility and lifecycle behavior and are excluded from this shortcut surface. Option D is incorrect because every shortcut invocation must be acknowledged promptly, regardless of how many shortcuts the app defines. Current Slack UX guidance requires shortcut requests to be acknowledged with an HTTP 200 response within three seconds. Option C is not a general rule; follow-up notification mechanics depend on the interaction payload and workflow. A's fixed limit of three message shortcuts is not the applicable platform rule represented here. This belongs to Design the Interactive Flow of Your App, particularly message shortcuts and contextual interactions.