After completing the sprint on an agile project, a team updates the risk burndown chart...

In agile projects, the risk burndown chart is used to monitor the reduction of project risk over time. If risk exposure is found to be increasing, it is essential to act immediately by selecting and implementing risk mitigation actions that can reduce exposure in upcoming iterations. The PMI “Agile Practice Guide” and the PMBOK® Guide state:

“In agile projects, risks are identified and managed continuously. When risk exposure is high, mitigation actions (often called ‘risk mitigation stories’) should be prioritized in the next iteration or sprint to bring exposure back to acceptable levels.”

— Agile Practice Guide, PMI, Section 5.7 (Responding to Risks in Agile Projects)

“Project teams may develop risk response actions as work items and prioritize these for execution in the backlog, particularly in iterative and incremental environments.”

— PMBOK® Guide, 6th Edition, Section 11.6.2.3

Therefore, prioritizing risk mitigation stories for the next sprint (Option C) is the most effective and officially recommended response.