The first blank can be filled with option B, that is, radiant energy, and the second blank can be filled with option C, that is, the Sun.
Radiant energy is most generally applied in the fields of solar energy, radiometry, lighting, and heating, however, is also in certain cases used in other branches like telecommunications. In modern uses, comprising the transmission of power from one location to another, radiant energy is also used to signify towards the electromagnetic waves. Thus, it is the form of energy that can move from place to place across the universe.
A surface heated by the Sun transforms the energy of the light into infrared energy, that is, a form of radiant energy, thus, the main source of radiant energy on Earth is the Sun.