The one of the following that is used to change the speed of certain types of electric motors is a(n):

The correct answer is related to the use of a rheostat, which is an adjustable resistor that can control the current flowing through a circuit. In the context of electric motors, a rheostat can be used to vary the voltage and thereby adjust the speed of certain types of motors, especially in small, low-power applications. This capability makes it particularly useful for applications where fine adjustments to motor speed are necessary.

In contrast, a commutator serves to reverse the direction of current in a motor's windings but does not control speed. A brush is a component that transfers electrical current to the moving part of a motor but does not regulate the speed either. The armature is the rotating part of an electric motor that carries the current, but like the others, it does not control speed. Thus, the rasp of the rheostat in managing flow and consequently motor speed is what makes it the most appropriate choice for this question.