Which of the following is not an example of a variable resistor?

The correct answer is that photostats are not examples of variable resistors. Photostats operate based on photoelectric principles, using light to initiate processes such as the reproduction of images or the registration of light intensity. While they can involve variable resistance due to their response to light, they do not function primarily as variable resistors in the same sense as the other options.

In contrast, potentiometers, thermistors, and rheostats are all designed explicitly to adjust resistance. Potentiometers allow for the manual adjustment of resistance and are frequently used in volume controls and other applications where variable voltage or current is needed. Thermistors are temperature-sensitive resistors and change resistance with temperature variations, making them crucial in temperature measurement and control systems. Rheostats are specifically designed for variable resistance and can handle larger currents, making them useful for controlling the brightness of lights or adjusting the speed of motors.

By understanding the specific functions and applications of these components, it is clear why photostats do not fit into the category of variable resistors in the same practical context as the others listed.