Helimutt,
Quite simply, your answer is a).
Put simply, it refers to rotor RPM (Nr). More collective pulled = more static droop. This is very apparent on the older AS-330J Puma(with a mechanical FCU using flyweights and needle-valves to control fuel flow) where the flat pitch Nr is higher than the hover Nr.
However, on later versions such as the AS-332 Super Puma the Makila FCU is more sophisticated. At flat pitch on the ground the Nr sits slightly low and is artificially increased by the FCU when the lever is raised for lift-off to the hover.
We can now all discuss which way is positive droop and which way is negative droop