The correct answer is A. Speech recognition requests will fall back to the most recent base model for the same locale . Microsoft's custom speech model lifecycle guidance states that when a model expires, it is no longer available for transcription. For the custom endpoint route, speech recognition requests fall back to the most recent base model for the same locale. The documentation also warns that recognition results might still be returned, but the transcription may no longer reflect the domain-specific adaptation of the custom model.
This distinguishes real-time custom endpoint behavior from batch transcription behavior. Batch transcription requests that specify an expired model fail with a 4xx error, but that is not the route described in this question.
The agent is using a custom endpoint for real-time recognition, so fallback to the latest base model is the expected behavior. The model is not automatically deleted merely because it expires, and it does not continue to use the expired custom model indefinitely. The operational recommendation is to update the endpoint's model before expiration by redeploying the endpoint with a newer custom model. Reference topics: Azure Speech custom speech model lifecycle, custom endpoints, model expiration, real-time speech recognition, and endpoint redeployment.