Which of the following instruments is used to measure temperature?

The instrument used to measure temperature is the thermometer. A thermometer functions based on the principle of thermal expansion, where a liquid (often mercury or colored alcohol) rises or falls in a glass tube in response to changes in temperature. This allows for precise measurements of thermal energy, which is essential in various scientific experiments and applications.

Other instruments, such as test tubes, spatulas, and stirring rods, serve different purposes in the lab. Test tubes hold liquids for mixing or heating but do not measure temperature. Spatulas are used for transferring solid chemicals and do not provide any measurement capability. Stirring rods are intended to mix solutions but also lack the ability to gauge temperature. The thermometer is specifically designed to provide accurate temperature readings, making it the correct choice for this question.