At which of the following temperatures is the average kinetic energy of the molecules of a substance the least? 23.2 °C 35.2 °C 15.2 °C 31.4 °C

At 15.2°C. Kinetic energy of molecules highly depends on the temperature - the warmer it is, the faster the molecules will move, especially in fluids (gases and liquids). If we consider that the formula for average kinetic energy of molecules is:

Ek = 3/2*k*T where k is Boltzmanns constant and 3/2 is, well, 3/2, kinetic energy of molecules really only depends on the temperature.