What must an app do shortly after receiving an Events API request to avoid retries?

Slack requires an application using HTTP Events API delivery to acknowledge an event request with an HTTP `2xx` response within three seconds. If Slack does not receive a successful acknowledgment in that period, the delivery attempt is treated as unsuccessful and Slack may retry the event. Slack recommends responding with `HTTP 200 OK` as rapidly as possible and separating event ingestion from potentially slow business processing. A robust production architecture typically acknowledges the request immediately, places the event on a queue, and performs downstream processing asynchronously. This prevents expensive API calls, database operations, or external-service dependencies from delaying acknowledgment and producing duplicate deliveries. Returning Block Kit content is unnecessary for acknowledgment, while user logout and scope requests are unrelated. Therefore, A is correct and represents an important Slack scalability and reliability pattern.