Answer is A & B since we are only talking on how events get published.
Events can be published by
1.) listening to a platform event using process builder or flow and use outbound messages.
2.) use apex to define the event instance and EventBus.publish
3.) use salesforce apis to create or publish an event. {{_endpoint}}/services/data/v54/sobjects/{{eventname}}
There's also many ways on how to subscribe to events externally but one way is to use cometD (Streaming API)

A, C
https://developer.salesforce.com/docs/atlas.en-us.234.0.platform_events.meta/platform_events/platform_events_publish.htm