Introduction:
In the field of process control, a control system typically consists of a process variable (PV), a measuring element, a controller, and a final control element (FCE). The FCE is responsible for manipulating the process variable to maintain it at the desired value. It can be any device or mechanism that adjusts the process input, such as a control valve, a servomotor, an electro-pneumatic converter, etc. However, in this case, the correct answer is option B: Potentiometer.

Explanation:
A potentiometer, also known as a variable resistor, is an electrical component with three terminals. It consists of a resistive element and a movable contact, which slides along the resistive track. By adjusting the position of the movable contact, the resistance between the terminals can be varied, allowing for control over the flow of electric current.

Control Valve:
A control valve is a common example of a final control element. It is used to regulate the flow of fluid (liquid or gas) in a process system. By adjusting the valve position, the flow rate can be controlled, thereby manipulating the process variable. Control valves are widely used in industries such as oil and gas, chemical, power plants, etc.

Electro-Pneumatic Converter:
An electro-pneumatic converter is another type of final control element. It converts an electrical signal into a proportional pneumatic output. This pneumatic output can then be used to control pneumatic actuators, such as cylinders or valves, in a process system. Electro-pneumatic converters are commonly used in industrial automation and control systems.

Servomotor:
A servomotor is a rotary actuator that allows for precise control of angular position. It typically consists of a motor, a position sensor, and a control circuit. By applying the appropriate control signals, the servomotor can be accurately positioned, making it suitable for applications that require precise motion control.

Potentiometer:
A potentiometer, as mentioned earlier, is not a final control element. It is primarily used as a variable resistor or as a voltage divider in electrical circuits. It is often used for measurement or control purposes, but it does not directly manipulate the process variable in a control system.

Conclusion:
In summary, the potentiometer is not a final control element in a control system. It is a variable resistor used for measurement or control purposes in electrical circuits. The final control elements in a control system are devices or mechanisms that directly manipulate the process variable, such as control valves, servomotors, or electro-pneumatic converters.