Explanation:

To determine how platform events can be published in a Salesforce org after they are defined, let's evaluate the provided options:

➝ A. External Apps use an API to publish event messages.
➝ B. Internal Apps can use entitlement processes.
➝ C. External Apps require the standard Streaming API.
➝ D. Internal Apps can use outbound messages.

Platform events in Salesforce are part of an event-driven architecture, allowing communication of changes or updates within Salesforce or with external systems. Once defined, these events need to be published so that subscribers (internal or external) can receive them. Let's analyze each option to identify the correct mechanism.

Option A: External Apps use an API to publish event messages
External applications, which operate outside of Salesforce, can interact with the platform through APIs. Salesforce provides the REST API and SOAP API, both of which support publishing platform events. For example, an external app can make a REST API call to create an event message, effectively publishing it to the Salesforce org. This is a well-documented and valid mechanism for external systems to publish platform events. Thus, this option appears correct.

Option B: Internal Apps can use entitlement processes
Internal apps refer to functionalities or applications within Salesforce. Entitlement processes, however, are a feature of Salesforce Service Cloud used to manage customer service entitlements, such as support timelines or service contracts. They are not designed for publishing platform events, which is a separate process involving event creation and distribution. This option does not align with the mechanisms available for publishing events, making it incorrect.

Option C: External Apps require the standard Streaming API
The Streaming API in Salesforce is designed for subscribing to real-time notifications, such as PushTopics or generic streaming events. For platform events specifically, subscribers use the CometD protocol to listen for events. However, publishing events is a distinct action that involves creating and sending event messages, not subscribing to them. The Streaming API is not used for publishing events; instead, publishing is handled through APIs (like REST or SOAP) or Apex. Therefore, this option is incorrect.

Option D: Internal Apps can use outbound messages
Outbound messages are a workflow action in Salesforce that send SOAP-based notifications to external systems when specific conditions are met, such as a field update. While they facilitate communication from Salesforce to external services, they are not a mechanism for publishing platform events within the Salesforce ecosystem. Platform events are published using tools like Apex, Process Builder, Flow Builder, or APIs, not outbound messages. This option is also incorrect.

Additional Context and Verification

For completeness, platform events can be published in several ways:
➜ Within Salesforce (internal): Using Apex (e.g., EventBus.publish), Process Builder, or Flow Builder.
➜ Externally: Using the REST API or SOAP API.

The question asks for a mechanism by which events "can be published," and among the options, only A correctly identifies a valid method: external apps using an API. While internal mechanisms like Apex or Flow Builder are also valid, they are not represented in the options. Options B, C, and D describe mechanisms that do not apply to publishing platform events.

Conclusion
The correct mechanism among the given choices is that external apps can use an API to publish event messages. Therefore, the answer is:
A. External Apps use an API to publish event messages.