The correct answer is: B. Sensitivity
Sensitivity is the ratio of the rate of change of discharge of an outlet to the rate of change of discharge of the parent channel. It is a measure of how responsive an outlet is to changes in the flow of water in the parent channel.
Efficiency is a measure of how well an outlet can deliver water to the downstream system. It is not related to the rate of change of discharge.
Flexibility is a measure of how well an outlet can be adapted to different flow conditions. It is not related to the rate of change of discharge.
Modular limit is the maximum discharge that an outlet can safely handle. It is not related to the rate of change of discharge.