The answer is C. Thermal energy is transferred to her face by radiation, and thermal energy is transferred to the bottoms of her feet by conduction.
Conduction is a method of transferring heat by using the collision of molecules and free electrons (depending on the material). Heat is usually transferred by conduction if both objects are in contact with each other that way the molecules can collide and spread the kinetic energy thus raising their internal heat. So, since the feet and the sand are in contact, so the heat should be transferred by conduction. However, air is a very poor heat conductor, it cannot transfer heat efficiently, therefore, the first answer is not conduction.
Radiation is a method of transferring heat too but using electromagnetic waves. It works great even in the air, or even with vacuum space. The sun shining from outer space to earth through the vacuum space is also by radiation. So, the first answer is radiation.
