The correct answer is: B. Nonlinear resistors.
A varistor is a two-terminal nonlinear resistor, with a large change in resistance with small changes in voltage. Varistors are commonly used as voltage limiters, surge suppressors, and transient absorbers.
Option A is incorrect because varistors are not insulators. They are resistors, which means that they conduct electricity.
Option C is incorrect because varistors are not carbon resistors. Carbon resistors are made of carbon, while varistors are made of a variety of materials, including zinc oxide, barium titanate, and manganese oxide.
Option D is incorrect because varistors do not have zero temperature coefficients. The resistance of a varistor changes with temperature, but the change is not linear.