Final answer:
Molarity is the concentration unit that varies with temperature because it depends on the volume of the solution, which changes with temperature. Other concentrations like molality, mole fraction, and mass percent are based on mass and do not change with temperature.
Step-by-step explanation:
The concentration unit that varies with temperature is molarity. Molarity is defined as the number of moles of solute divided by the volume of the solution in liters. Since the volume of a solution can change with temperature due to the expansion or contraction of the solution, the molarity can vary. Conversely, other concentration units such as molality, mole fraction, and mass percent are based on mass, which does not change with temperature. Therefore, chemists often opt for molality over molarity for temperature-dependent studies since molality is based on the kilograms of solvent and does not change with temperature.