We cannot use mercury thermometer at low temperatures because:
A.)Glass might break down at low temperature.
B.)Heat does not flow from the body whose measurement we are taking with the thermometer.
C.)At low temperatures mercury becomes transparent and it becomes difficult to take the readings.
D.)Mercury freezes at low temperatures.

Hint: Mercury has a freezing point at \[\text{-37}{}^\circ \text{C}\]. For every material, freezing point makes the liquid as solid. For the thermometer applications, liquids have to be freely-flowing. Otherwise, it will show the wrong measurements.

Complete step by step answer: Even though mercury thermometers have a wide range of temperature measurement \[(-\text{37 to 356}{}^\circ \text{C)}\], it can’t be used for the lower temperatures. As we know, the mercury will solidify itself at \[-38.83{}^\circ \text{C}\]. For the extension of temperature range, inert gas like nitrogen will add into the mercury. So, when the mercury becomes solid at lower temperatures, the nitrogen gas will get trapped. When we reuse this thermometer for the high-temperature measurement, it will behave like an unstable thermometer. So, we have to repair this thermometer for further use if we have used a mercury thermometer at lower temperatures. It is very difficult to use mercury thermometers at a lower temperature since the mercury solidifies at lower temperatures. Hence the correct option is D.

Additional information:
Mercury is still used for the preparation of thermometers due to the various physical and chemical properties of mercury.

a.)It is considered as a rapid heat conductor.
b.)It allows linear expansion.
c.)No capillary force occurs between the glass tube and mercury.
d.)Mercury does not wet the glass tube surface. So the measurement will be very accurate.
e.)It has a high boiling point \[\text{(357}{}^\circ \text{C)}\]

Applications of mercury thermometer are;

a.)Clinical measurements
b.)Laboratory purposes
c.)Room temperature measurements

Even though mercury vapour is a poisonous substance.

Note: When mercury is freezing, its density slightly increases. Solid mercury is considered as very ductile and malleable material. It can be cut with a simple knife. Mercury will become transparent only if it is a vapour state. At vapour state, the mercury will be transparent, odourless and toxic vapour.