Which component emits a voltage when heated or cooled?

The component that emits a voltage when heated or cooled is a thermocouple. A thermocouple consists of two dissimilar metals joined at one end. When there is a temperature difference between the junction where the two metals meet and the other ends (which are connected to a measurement device), a voltage is generated due to the Seebeck effect. This effect arises from the different electrical properties of the two metals, causing a measurable voltage that correlates to the temperature difference.

In contrast, a diode is primarily used for controlling the flow of current in one direction rather than for temperature sensing or generating voltage based on temperature changes. A capacitor serves the purpose of storing and releasing electrical energy, while a transistor is used for amplification and switching of electronic signals. Neither of these components generates a voltage in response to temperature variations, which further emphasizes why the thermocouple is the correct choice for this question.