After a platform event is defined in a Salesforce org,events can be published via which...

Platform events in Salesforce can be published using a variety of mechanisms, including from external applications through the Salesforce APIs.

Option A is correct because external applications can use Salesforce APIs such as the REST API or the SOAP API to publish events to a platform event in Salesforce.

Options B, C, and D describe mechanisms that are not used for publishing platform events. Entitlement processes are for setting up and tracking service contracts, the standard Streaming API is for subscribing to events not publishing them, and outbound messages are a workflow action used to send information to external services, not for publishing platform events.