Per the AgentForce Deployment and Flow Integration Guide, when deploying flows via change sets, the flows arrive in the production org in "inactive" status by default. The administrator must manually activate the flow post-deployment before it can be executed by agents or users.
This explains why the agent encounters an error when attempting to run the flow-the system recognizes the flow but cannot invoke it because it remains inactive.
Option B is incorrect since permission errors would display an "insufficient privileges" message, not a runtime failure. Option C is unlikely because dependent Apex classes would be automatically handled if properly included in the deployment.
Therefore, the most likely cause is Option A - The flow was not manually activated in production after deployment.
Reference: AgentForce Deployment Guide - "Post-Deployment Flow Activation Requirements."