ANS:A - Density

The property of air that does not increase with an increase in temperature is density. According to the ideal gas law, as temperature increases, the density of a gas decreases, assuming pressure and mass remain constant. This is because the average kinetic energy of gas molecules increases with temperature, causing them to move faster and spread out more, leading to a decrease in density. On the other hand, the other properties mentioned - thermal diffusivity, viscosity, and thermal conductivity - generally do increase with an increase in temperature:

• Thermal diffusivity: This property measures the rate at which heat diffuses through a material relative to its ability to store heat. In general, thermal diffusivity tends to increase with temperature, particularly in gases like air.
• Viscosity: Viscosity refers to a fluid's resistance to flow. In gases, viscosity typically increases with temperature due to increased molecular motion, leading to stronger intermolecular forces and greater resistance to flow.
• Thermal conductivity: Thermal conductivity represents a material's ability to conduct heat. In gases like air, thermal conductivity generally increases with temperature as higher temperatures lead to increased molecular collisions and energy transfer, enhancing the material's ability to conduct heat.