The purpose of a Key Control Indicator (KCI) is to measure control effectiveness. Therefore, the most important design criterion is that it demonstrates whether the control objective is being achieved.
ISACA defines:
"KCIs are metrics that measure the effectiveness of controls in achieving their intended objectives." Quantitative methods are desirable but secondary.
Hence, B is correct.
CRISC Reference: Domain 4 - Risk and Control Monitoring and Reporting, Topic: Control Performance Metrics.