Hint: A thermometer is a device that is used to measure the temperature of the body. The word thermometer is made of two words- ‘thermo’ and ‘meter’. The word ‘thermo’ means ‘heat’ and ‘meter’ means ‘to measure’. The temperature is the measure of hotness or coldness of a body. The temperature of a body is measured in units ‘Celsius’.
Complete answer: The liquid used in clinical thermometers is mercury. It is widely used because it has a shiny appearance and it helps to measure the temperature very accurately. It has a very high boiling point due to which it is considered suitable for measuring bodies that have high temperatures. Also it does not stick to the surface of the glass tube. It has a constant coefficient of expansion, because of which it expands uniformly at all the temperatures.
Hence, the correct answer is option (D).
Note: It is to be noted that the coefficient of thermal expansion explains that if the temperature of a body changes then how it changes the size of the object. When a substance is heated, then its temperature increases. The molecules start vibrating at a faster speed and occupy comparatively high volume. This means that the substances expand on heating and contract on cooling. The coefficient of thermal expansion can be written as a fractional increase in length per unit rise of temperature of the body. But mercury is also toxic, so nowadays mercury is not much used in thermometers.