The correct answer is A (Earned Value) because, within the NCMA Contract Management Body of Knowledge (CMBOK), Earned Value Management (EVM) is a key performance measurement technique used to assess both cost and schedule performance of a project. It integrates scope, schedule, and cost data to provide an objective measure of project progress.

Earned Value compares three primary metrics: Planned Value (PV), which represents the work that was scheduled; Earned Value (EV), which represents the work actually accomplished; and Actual Cost (AC), which reflects the cost incurred. By analyzing these metrics, contract managers can determine whether the project is ahead of or behind schedule and under or over budget.

This method is particularly valuable in contract management because it provides early warning indicators of performance issues, allowing corrective actions to be taken before problems escalate. It is widely used in both government and commercial contracting environments to ensure accountability and effective performance monitoring.

Option B (Baseline) refers to the approved plan against which performance is measured but is not the measurement method itself. Option C (Estimate at Completion) is a forecasting tool derived from earned value data. Option D (WBS) is a planning tool used to define scope, not measure performance.

Thus, Earned Value is the correct and comprehensive method for evaluating project performance in CMBOK.