The answer to this is A.
Heat transfer by conduction only happens when the two objects are in contact in each other. This is because conduction relies on the collision of molecules of the two object.
When an object has a higher temperature, their molecules will have a higher kinetic energy. They move faster and eventually collide into other slower molecules, then spreading their energy out and making those molecules move faster and have a higher kinetic energy too. Both object will now have the same temperature (ie. Thermal equilibrium is achieved).
Therefore, as the air molecules touches the ground, they collide to the ground molecules and therefore warming it up. The answer is A.
