The correct answer is 'Uptime' as it directly refers to the amount of time a service or system is operational and available for use, which is a key performance indicator for service availability. 'Downtime' measures the time when the service is not operational, 'Latency' measures the delay before a transfer of data begins following an instruction, and 'Throughput' measures the volume of data that can be processed by a system or network over a given period.