For heat transfer, there's basically only convection, conduction, and radiation. So first we eliminated A.
Next, we need to know that the space between the sun and the earth is mainly vacuum, not air. Therefore, there's no fluid flow happening and convection may not be the right choice.
Then, we also need to know that vacuum is the worst conductor of heat in conduction, which is also the best insulator or heat in another way. Therefore the rate of conduction is so low that we can also eliminate choice C.
Now only remain B, radiation. Radiation is possible in vacuum and it is always happening even if we don't see it.
