The correct answer is: A. thermometer.
A thermometer is a device that measures temperature. It is a sensor because it detects and responds to a physical change in the environment, in this case, a change in temperature. The thermometer then converts this change into a measurable signal, such as a reading on a scale.
A brake is a device that is used to slow or stop a moving object. It is not a sensor because it does not detect or respond to a physical change in the environment. Instead, it is a mechanical device that is used to apply force to a moving object in order to slow or stop it.
A relay is an electrical device that is used to control a circuit by opening or closing a switch. It is not a sensor because it does not detect or respond to a physical change in the environment. Instead, it is an electrical device that is used to control the flow of electricity in a circuit.
A clutch is a mechanical device that is used to connect and disconnect two rotating shafts. It is not a sensor because it does not detect or respond to a physical change in the environment. Instead, it is a mechanical device that is used to connect and disconnect two rotating shafts.
Therefore, the only device that could be part of a sensor is a thermometer.