What are the markings on a mercury-in-glass thermometer designed to indicate?

The markings on a mercury-in-glass thermometer are designed to indicate the temperature based on the length of mercury in the tube. As the temperature rises, the mercury expands and moves up the tube. The height of the mercury column corresponds to a specific temperature measurement, which is calibrated on the scale printed on the glass. Because mercury has a consistent expansion rate, the thermometer can provide accurate readings of temperature changes as it correlates directly with the length of the mercury column observed. This is why the correct answer focuses on the length of mercury facilitating an understanding of temperature rather than pressure, volume, or density, which are unrelated to the primary function of a thermometer.