The Fed Funds Rate Meets the Requirement for an Operating

The fed funds rate meets the requirement for an operating target. Which of the below is this requirement?

A) The rate is readily observable and continuously available because the market for fed funds is large and active.
B) The rate is readily observable and continuously available because the market for fed funds is small but active.
C) The rate is readily observable and continuously available because the market for fed funds is large but inactive.
D) The rate is often not observable or available because the market for fed funds is small and/or inactive.