The correct answer is D. sensitivity.
Sensitivity is the ratio of the rate of change of the output of a system to the rate of change of its input. In the case of a distributing channel, the output is the discharge of the outlet, and the input is the discharge of the distributing channel. Therefore, the sensitivity of a distributing channel is the ratio of the rate of change of the discharge of the outlet to the rate of change of the discharge of the distributing channel.
A. proportionality is the relationship between two quantities in which the change in one quantity is always in direct proportion to the change in the other quantity. In the case of a distributing channel, proportionality would mean that the discharge of the outlet would always change in direct proportion to the change in the discharge of the distributing channel. However, this is not always the case, as the discharge of the outlet can also be affected by other factors, such as the size of the outlet and the head of water.
B. flexibility is the ability of a system to respond to changes in its environment. In the case of a distributing channel, flexibility would mean that the channel would be able to adjust its discharge to accommodate changes in the demand for water. However, this is not always the case, as the discharge of a distributing channel can be limited by factors such as the size of the channel and the head of water.
C. setting is the value of a parameter that is used to control the operation of a system. In the case of a distributing channel, the setting could be the value of the head of water that is used to control the discharge of the channel. However, the setting is not always the same, as it can be adjusted to meet the changing needs of the system.