For what purpose is a transistor commonly used in electronic circuits?

Transistors are fundamental components in electronic circuits, primarily utilized for their ability to amplify and switch electronic signals. When used as an amplifier, a small input current or voltage can control a larger output current or voltage, making transistors essential in audio and radio frequency applications where signal strength is crucial. In switching applications, transistors can toggle between on and off states, serving as electronic switches. This functionality allows them to control high power devices and is vital in digital circuits, logic gates, and microprocessors.

The other choices, while they may involve electrical concepts, do not accurately capture the primary purpose of transistors. Restricting current flow is more related to resistors or specific circuit designs rather than the intrinsic function of transistors. Converting AC to DC pertains more to rectifiers, while measuring resistance is the role of ohmmeters. Each of these functions plays a distinct part in electronics, but they do not align with the core capabilities of transistors.