B). Use a fallback topic: Power Virtual Agents provides the capability to handle unrecognized inputs by using fallback topics. A fallback topic is a topic that is triggered when the bot is unable to recognize the user input.
You can configure fallback topics by going to the Power Virtual Agents portal, and then select the bot you want to configure. Then select the "Topics" tab and create a new topic with a fallback trigger. Reference:
https://docs.microsoft.com/en-us/power-virtual-agents/topics-triggers#fallback-triggers C). Display a system-defined error message: This is another option to handle unrecognized inputs by displaying a predefined message that inform the user that the bot was unable to understand their input.