Which component is the smallest in size and can amplify electrical signals?

The smallest component in size that can amplify electrical signals is the transistor. A transistor is a semiconductor device that can control the flow of electrical current and amplify signals, making it vital in various electronic devices for functions such as switching and signal modulation.

In contrast, a relay functions as an electrically operated switch but is larger and primarily used for switching circuits rather than amplifying signals. A resistor is used to limit current and divide voltages but does not amplify signals; it instead dissipates energy as heat. A capacitor stores electrical energy temporarily for use in smoothing out voltage fluctuations and timing applications, but it does not amplify signals either. Thus, recognizing the unique role of a transistor as both a compact size component and an amplifier distinguishes it from the other options in the list.