Explanation:

Heat conduction in gases is primarily due to the elastic impact of molecules. When a gas is heated, the molecules gain kinetic energy and move faster. These fast-moving molecules collide with slower-moving molecules, transferring their kinetic energy to them. This transfer of energy through molecular collisions is known as heat conduction.

The motion of electrons and the mixing motion of different layers of the gas also play a role in heat conduction in gases, but they are not the primary mechanisms. Let's explore each option in detail:

a) Electromagnetic waves: While electromagnetic waves can transfer energy, they are not the main mechanism for heat conduction in gases. Electromagnetic waves primarily transfer energy through radiation, which is the dominant mode of heat transfer in vacuum or transparent materials.

b) Motion of electrons: The motion of electrons contributes to the conduction of heat in solids and metals, where electrons can move freely. However, in gases, the motion of electrons is limited due to their lower density and higher distances between atoms or molecules. Therefore, the motion of electrons is not the main mechanism for heat conduction in gases.

c) Mixing motion of different layers of the gas: The mixing motion of different layers of the gas, known as convection, can contribute to heat transfer in gases. Convection occurs when hot gas rises and cold gas sinks, creating circulation patterns. However, convection is more relevant to bulk heat transfer in gases, such as in natural or forced convection, rather than heat conduction within a gas.

d) Elastic impact of molecules: The elastic impact of molecules is the primary mechanism for heat conduction in gases. As mentioned earlier, when a gas is heated, the molecules gain kinetic energy and move faster. These fast-moving molecules collide with slower-moving molecules, transferring their kinetic energy. This transfer of energy through molecular collisions allows heat to propagate through the gas.

In conclusion, while other mechanisms such as electromagnetic waves, electron motion, and convection can contribute to heat transfer in gases, the primary mechanism for heat conduction in gases is the elastic impact of molecules.