A key control indicator (KCI) is a metric that provides information on the extent to which a given control is meeting its intended objectives in terms of loss prevention, reduction, etc. A KCI should have an explicit relationship to both the specific control and the specific risk against which the control has been implemented.
For risk related to IT infrastructure failure, a possible control is to have a recovery plan that can restore the critical IT services and minimize the impact of the failure. A KCI that can measure the effectiveness of this control is the number of successful recovery plan tests, which indicates how well the recovery plan can be executed in a real scenario. The higher the number of successful tests, the lower the risk of IT infrastructure failure. Therefore, this is the best KCI among the given options. References = Integrating KRIs and KPIs for Effective Technology Risk Management Key Control Indicator (KCI) - CIO Wiki Infrastructure Issues: Understanding and Mitigating Risks