The correct answer is A. Disturbance.
A disturbance is a signal other than the reference input that tends to affect the value of controlled variable. It is an unwanted input that can cause the system to deviate from its desired state.
A command is a signal that specifies the desired value of the controlled variable. It is also known as the reference input or setpoint.
A control element is a device that is used to manipulate the manipulated variable in order to achieve the desired value of the controlled variable.
A reference input is a signal that specifies the desired value of the controlled variable. It is also known as the command or setpoint.
Here is an example of a disturbance:
Suppose you are trying to control the temperature of a room using a thermostat. The thermostat is the control element, and the temperature of the room is the controlled variable. The reference input is the desired temperature of the room. The disturbance is anything that affects the temperature of the room other than the thermostat, such as opening a window or turning on a heater.
If a window is opened, the temperature of the room will decrease. The thermostat will detect this decrease and turn on the heater to compensate. However, if the heater is not powerful enough, the temperature of the room will continue to decrease. This is an example of how a disturbance can cause the system to deviate from its desired state.