The correct answer is: C. The time it takes for the mass of the process to respond to an input change.
Process lag is the time it takes for the mass of the process to respond to an input change. This is because the mass of the process has inertia, which means that it takes time for it to start moving or stop moving. The amount of process lag depends on the mass of the process and the rate of change of the input.
Option A is incorrect because the time it takes for the sensor to respond to a change is called sensor lag. Option B is incorrect because the lag in the error signal when the setpoint is changed is called control lag. Option D is incorrect because not all of the above are correct. Option E is incorrect because C is the correct answer.