In which scenario would platform events be most appropriate?

Platform events are designed specifically for scenarios that require the publication and subscription of event messages, regardless of whether those events occur within Salesforce or in external systems. This model facilitates real-time communication and integration, allowing various systems to respond to events as they happen.

When you publish a platform event, it can be consumed by different consumers, whether they are other Salesforce processes, external applications through APIs, or even end-user interfaces. This multichannel capability makes platform events particularly well-suited for integrations, enabling dynamic interactions and workflows that react to events as they occur in real-time.

The other scenarios, while relevant to Salesforce functionality, do not align with the core use case of platform events. For instance, scheduling a calendar event focuses on a specific type of task management rather than event-driven communication. Broadcasting messages with a payload containing only the message body seems too simplistic since platform events generally encapsulate more detailed data and are structured rather than a single-sided communication. Similarly, using Apex and Streaming API does not inherently leverage the specialized design intent of platform events aimed at broad inter-system event publishing and subscription.

Thus, opting for the scenario where platform events are utilized fully encapsulates their purpose and functionality within Salesforce's broader data and application ecosystem.