The temperature of a gas is indeed a measure of the average kinetic energy of its molecules. This is a principle that comes from kinetic molecular theory, which helps us understand the behavior of gases. According to this theory, temperature is directly proportional to the average kinetic energy of the gas molecules. This means that as the temperature increases, the average kinetic energy of the gas molecules also increases, and vice versa. The correct answer is therefore: Option A: the average kinetic energy of gas molecules. Here's why the other options are incorrect: Option B: The temperature of a gas does not measure the average potential energy of gas molecules. Potential energy in the context of gas molecules often refers to the energy associated with the positions of the molecules relative to each other (e.g., due to intermolecular forces), which is not what temperature measures. Option C: The temperature of a gas is not a measure of the average distance between the molecules of a gas. While temperature can affect the volume of gas (and thus indirectly influence average distances between molecules in some contexts), the temperature itself is specifically a measure of kinetic energy, not spatial distribution or distances between molecules. Option D: The temperature of a gas does not measure the size of the molecules of a gas. Molecular size is related to the type of gas and its molecular structure, not to the temperature of the gas.