At constant temperature, the thermal conductivities of gases __________ with rise in pressure.

ANS:A - decrease

At constant temperature, the thermal conductivities of gases generally increase with the rise in pressure. This is because at higher pressures, the mean free path of gas molecules decreases due to increased molecular interactions. As a result, the collisions between gas molecules become more frequent, facilitating faster energy transfer through the gas. Consequently, the thermal conductivity, which measures the ability of a material to conduct heat, tends to increase with increasing pressure in gases. Therefore, the correct option is: increase.