Which of the following is measured by a thermocouple?

A thermocouple is a device used to measure temperature. It operates based on the thermoelectric effect, where two different metals are joined together and produce a voltage when there is a temperature difference between the junctions. This voltage can be correlated to temperature values, allowing for precise temperature measurements in various applications, such as in engines, ovens, or industrial processes.

The other options deal with different parameters. Velocity is associated with the speed of an object, pressure measures the force exerted by a fluid, and humidity refers to the amount of moisture in the air. Each of these requires specific types of sensors or instruments, such as an anemometer for velocity, a manometer or pressure transducer for pressure, and a hygrometer for humidity, none of which are capable of measuring temperature like a thermocouple does.