The answer is (a) Operating Speed.
With respect to a gate:
Propagation delay: Time required for the gate to change its state measured from the instant at which input is applied to the gate.
Operating speed: Maximum frequency at which digital data can be applied to the gate.
Noise immunity: Amount of noise that can be applied to the input of a gate without causing the gate to change state.
Courtesy: Pearson question bank for Electronics and Communication Engineers by Satish K. Karna
