To trigger and receive campaign decisions from a server-side call to the Event API, the request payload must identify the event's origin. This is accomplished by including the source object in the event payload and setting its channel property to "Server". This signals to Personalization that the interaction is happening on a server, making it eligible for server-side campaigns, as opposed to web or mobile channels which are handled differently.

B. The attribute Server Side messages with a value of 1 is not a valid parameter in the Event API payload.

C. Server-side campaigns are not returned automatically; the event source must be correctly specified in the API request to be eligible.

D. The attribute Server side with a value of true is not the correct parameter for requesting server-side campaign responses.

Salesforce Help, Marketing Cloud Personalization, Use the Event API for Server-Side Campaigns, "To receive a server-side campaign response, you must include the source object in your event, and set the channel property to Server."