Which of the following plans should a project manager implement if a selected risk strategy fails to be fully effective?

A fallback plan is specifically defined as an alternative set of actions and tasks prepared in advance for use when the selected risk response strategy does not adequately address the risk. It is triggered by a defined condition in which the primary strategy proves insufficient, providing the project manager with a ready response to residual or secondary risk exposure.

A risk response plan documents all planned responses to identified risks and is the primary strategy document itself - it is what fails to be fully effective, not the backup to it.

'Mitigation management plan' is not a standard PMI-defined risk management document and does not appear in the PMBOK risk management framework.

A mitigation plan is a specific risk response strategy designed to reduce the probability or impact of a risk before it occurs - it is a primary response strategy, not the fallback when a strategy fails.