Correct answer is (a) Climb velocity
For explanation: The momentum equation for the helicopter in flight condition is given by:
T=\dot {m}(V+w)-\dot {m}V
T=\dot {m}w
Where, V is the climb velocity
w is the induced velocity in the wake region
T is the thrust
\dot {m} is mass flux
On observing the equation, we see that the momentum equation is independent of the climb velocity.