What is commonly used to measure temperature?

The thermometer is specifically designed to measure temperature, making it the correct choice. It operates based on various principles depending on the type, such as the expansion of liquids, changes in electrical resistance, or thermoelectric effects. Thermometers can be found in various forms, including mercury, digital, and thermocouple, each suitable for different temperature ranges and environments.

Other instruments listed, such as pressure gauges, Bourdon tubes, and diaphragms, serve different purposes. Pressure gauges are used to measure the pressure of gases or liquids, Bourdon tubes are components of pressure gauges that translate pressure changes into rotational movement, and diaphragms are typically used in pressure sensing applications or for separating fluids in instrumentation systems. None of these devices are intended for measuring temperature, which is why they are not suitable for this question.