The correct answer is C. Semiconductors.
Semiconductors are materials that have electrical conductivity much less than most of the metals but much greater than that of typical insulators. They are used in a wide variety of electronic devices, including transistors, diodes, and solar cells.
Varistors are devices that have a large change in resistance with a small change in voltage. They are used in surge protection and voltage limiting applications.
Thermistors are devices that have a large change in resistance with a small change in temperature. They are used in temperature sensing and control applications.
Variable resistors are devices that have a variable resistance. They are used in volume controls, dimmers, and other applications where the resistance needs to be adjusted.