The question asks about the instrument used to measure body temperature, and four options are provided: Lactometer, Calorimeter, Thermometer, and Barometer. Let's evaluate each option:
- Lactometer: This instrument is used to measure the purity or specific gravity of milk, not body temperature.
- Calorimeter: This device measures the heat of chemical reactions or physical changes, as well as heat capacity. It is not used to measure body temperature.
- Thermometer: This is a device specifically designed to measure temperature. There are various types of thermometers, such as mercury, digital, and infrared thermometers, used for measuring body temperature.
- Barometer: This instrument measures atmospheric pressure, which is unrelated to measuring body temperature.
Based on the given options, the Thermometer is the correct instrument used for measuring body temperature.
To conclude, the correct answer is: Thermometer.