What is the thermometric property of mercury used in thermometers?

The question asks about the thermometric property of mercury. A thermometric property is a physical property that changes measurably and predictably with temperature, allowing it to be used for temperature measurement.

Mercury-in-glass thermometers work based on the principle of thermal expansion. As the temperature of the mercury changes, its volume changes significantly and uniformly. This change in volume causes the mercury column to rise or fall in a narrow capillary tube, which is then calibrated to a temperature scale.

Let's evaluate the options:

• (A) resistance with temperature: While mercury is a conductor and its resistance changes with temperature, this is not the primary property utilized in standard mercury thermometers. Resistance thermometers use materials like platinum.
• (B) pressure with temperature: This property is typically used in gas thermometers, where the pressure of a gas at constant volume changes with temperature. It's not the basis for mercury thermometers.
• (C) density with temperature: Density is mass per unit volume. As the volume of mercury changes with temperature, its density also changes. However, the direct and most observable property used for measurement in a thermometer is the change in volume, not density directly.
• (D) Volume with temperature: This is the correct thermometric property. The expansion or contraction of mercury's volume with temperature is what makes it suitable for use in thermometers.

The final answer is $\boxed{\text{D) Volume with temperature}}$.