The correct answer is: C. The range of measured variables through which the air output changes from maximum to minimum.
The proportional band is a measure of how sensitive a controller is to changes in the measured variable. It is defined as the range of measured variables through which the air output changes from maximum to minimum. A wider proportional band means that the controller is less sensitive to changes in the measured variable, while a narrower proportional band means that the controller is more sensitive to changes in the measured variable.
Option A is incorrect because it refers to the range of air output, not the range of measured variables. Option B is incorrect because it refers to the range of measured variables from set value, not the range of measured variables through which the air output changes from maximum to minimum. Option D is incorrect because it is not a complete list of the possible answers. Option E is incorrect because it is not the correct answer.