What Is Measured by the Accumulated Benefit Obligation

What is measured by the accumulated benefit obligation?

A) The pension expense,computed by the plan formula applied to years of service to date,assuming future salary levels
B) The pension expense,computed by the plan formula applied to years of service to date,using existing salary levels
C) The pension obligation,computed by the plan formula applied to years of service to date,assuming future salary levels
D) The pension obligation,computed by the plan formula applied to years of service to date,using existing salary levels