The percentage of system availability is the most important key performance indicator (KPI) to establish in the service level agreement (SLA) for an outsourced data center. This KPI measures the uptime or reliability of the systems hosted by the data center provider, and reflects the ability of the provider to meet the customer’s expectations and requirements for system performance and accessibility. A high percentage of system availability indicates that the provider is delivering consistent and quality service, while a low percentage of system availability indicates that the provider is experiencing frequent or prolonged system failures or disruptions, which can negatively affect the customer’s business operations and reputation. Therefore, the percentage ofsystem availability is a critical factor for evaluating the effectiveness and efficiency of the data center provider, and should be clearly defined and monitored in the SLA. The other options are not the most important KPIs to establish in the SLA for an outsourced data center, as they do not directly measure the quality or reliability of the service provided. The percentage of systems included in recovery processes is a measure of the scope or coverage of the disaster recovery plan (DRP) of the data center provider, but it does not indicate how well the provider can execute the DRP or restore the systems in the event of a disaster. The number of key systems hosted is a measure of the capacity or utilization of the data center provider, but it does not indicate how efficiently or securely the provider can manage the systems. The average response time to resolve system incidents is a measure of the responsiveness or agility of the data center provider, but it does not indicate how effectively or proactively the provider can prevent or mitigate system incidents. References = Risk and Information Systems Control Study Manual, 7th Edition, Chapter 3, Section 3.2.3.4, Page 140.