The correct answer is D.
A proportional plus integral controller can compensate for lag and increase the system type.
A proportional controller only responds to the error signal, which is the difference between the setpoint and the process variable. This can cause the system to oscillate, as the controller tries to correct the error too quickly. An integral controller responds to the integral of the error signal, which smooths out the response of the system and eliminates oscillations.
A proportional plus integral controller can also increase the system type. The system type is a measure of how well the system responds to changes in the setpoint. A first-order system has a type of 1, a second-order system has a type of 2, and so on. A proportional plus integral controller can increase the system type by one, which makes the system more responsive to changes in the setpoint.
Option A is incorrect because a proportional plus integral controller does have a transient response. The transient response is the time it takes for the system to reach a steady state after a change in the setpoint. A proportional plus integral controller has a slower transient response than a proportional controller, but it is still faster than a pure integral controller.
Option B is correct because a proportional plus integral controller can compensate for lag. Lag is a type of time delay in the system. A proportional controller cannot compensate for lag, but a proportional plus integral controller can.
Option C is correct because a proportional plus integral controller can increase the system type. The system type is a measure of how well the system responds to changes in the setpoint. A proportional plus integral controller can increase the system type by one, which makes the system more responsive to changes in the setpoint.
Option E is incorrect because a proportional plus integral controller can do all of the things listed in options A, B, and C.