Which component acts as the transducer in a radio transmitter?

In the context of a radio transmitter, the component that acts as the transducer is the microphone. A transducer is a device that converts one form of energy into another. When it comes to a radio transmitter, the microphone takes sound waves (acoustic energy) and converts them into electrical signals (electrical energy). This electrical signal can then be modulated and transmitted via radio waves.

While other components listed have their respective functions—such as the antenna, which radiates or receives radio waves; the speaker, which converts electrical signals back into sound; and the transistor, which amplifies signals—they do not perform the transduction of sound into electrical signals. The microphone is specifically designed for this purpose, making it the correct answer in this context.