When controller output is proportional to the amount of deviation, the controller is:

A controller that has its output proportional to the amount of deviation from a set point is referred to as a proportional controller. This means that as the deviation increases, the output signal to the control element (such as a valve or compressor) also increases, maintaining a direct relationship between the input error and the controller's output.

This behavior allows for a more controlled and gradual response to changes, helping to minimize oscillations or overcorrections that can occur in other types of controllers. Proportional control is fundamental in many control systems because it provides a simple yet effective means of achieving stable operation in response to changes in the system.

The other options involve different control strategies. A proportional plus reset plus rate controller, for instance, integrates both proportional control and adjustments for accumulated errors (reset) and the rate of change to enhance the performance, indicating its complexity beyond simple proportional control. A two-position controller operates simply by switching on or off based on whether the deviation exceeds a certain threshold, leading to less nuanced control. An integral controller focuses solely on eliminating the steady-state error by considering the accumulation of past errors over time, which is also more complex than mere proportional control.