A transistor is controlled by the polarity and current at ________.

A transistor operates as a semiconductor device that regulates the flow of electrical current. The correct answer, which focuses on the base, highlights the crucial role this terminal plays in controlling the transistor's operation. The base of a bipolar junction transistor (BJT) is the region that receives input current, and its polarity influences the transistor's ability to conduct between the collector and emitter.

When a sufficient current flows into the base, it allows current to flow from the collector to the emitter, effectively turning the device on. The amount of current at the base controls how much current is allowed to flow through the collector and emitter paths, thereby acting as a switch or amplifier depending on the circuit configuration. Without proper base current and polarity, the transistor will not switch on properly.

The other alternatives, such as the collector, primarily serve as the output for the transistor and do not control its operation directly. While the emitter can influence current flow, it is the current and voltage at the base that primarily dictate the function of the transistor. Therefore, the base is central to controlling the transistor's behavior, making it the correct answer in this context.