The correct option is MTD. MTD stands for Maximum Tolerable Downtime and it is the metric that defines the longest period a system or service may be unavailable before severe or unacceptable business impact occurs.

The MTD is a business impact driven limit and it sets the outer bound that recovery plans and objectives must meet. Planned recovery objectives such as RTO are target times to restore service and they should be at or below the MTD. The RPO defines acceptable data loss windows and it measures how much data can be lost rather than how long a system can remain offline.

The RPO option is incorrect because it refers to the recovery point objective which measures acceptable data loss in terms of time since the last recoverable state. It does not express the maximum allowed downtime before severe business impact.

The RSL option is incorrect because it is not the standard business continuity term for the maximum allowable outage. The acronym may appear in some contractual or service level contexts but it is not the established metric used to define maximum tolerable downtime.

The RTO option is incorrect because it is the recovery time objective that defines the planned target to restore a system during an incident. It is a recovery goal within procedures and not the definition of the maximum tolerable outage itself.

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