**Explanation:**

The average kinetic energy and thermal energy are proportional to the absolute temperature (option D).

**Average Kinetic Energy:**

The average kinetic energy of particles in a substance is directly related to its temperature. The kinetic theory of gases states that the temperature of a gas is a measure of the average kinetic energy of its particles. This means that as the temperature increases, the average kinetic energy of the particles also increases.

**Thermal Energy:**

Thermal energy is the total kinetic energy of all the particles in a substance. It includes the kinetic energy of both the individual particles and their collective motion. As the temperature of a substance increases, the thermal energy also increases because the average kinetic energy of the particles increases.

**Relationship with Absolute Temperature:**

The absolute temperature of a substance is measured in Kelvin (K) and is directly proportional to the average kinetic energy of the particles. The relationship between absolute temperature and average kinetic energy is given by the equation:

Average Kinetic Energy ∝ Absolute Temperature

This means that as the absolute temperature increases, the average kinetic energy of the particles also increases. Consequently, the thermal energy of the substance increases.

**Explanation of Other Options:**

a) Specific Heat: Specific heat is the amount of heat energy required to raise the temperature of a substance by a certain amount. It is related to the thermal energy of a substance but is not directly proportional to the average kinetic energy or absolute temperature.

b) Volume: The volume of a substance does not directly affect the average kinetic energy or thermal energy of its particles. It is related to the density and mass of the substance.

c) Pressure: Pressure is the force exerted per unit area by a substance. It is related to the average kinetic energy of the particles, but it is not directly proportional to the absolute temperature. The relationship between pressure and temperature is described by the ideal gas law, which includes the volume and number of particles as well.

Therefore, the correct answer is option D, the average kinetic energy and thermal energy are proportional to the absolute temperature.