The correct answer is - A) atmosphere.
The Earth's atmosphere performs many roles that directly effect the conditions on the surface on the planet. One of the things that the atmosphere does is the reflection of big portion of the Sun's radiation. Each of the layers has its own role in the reflection, though some much more than the others. The stratosphere for example is the one that reflects the most of the Sun's radiation, and one of the main reasons for that is the layer in it, the ozone layer, which manages to reflect very large portion of the ultraviolet rays. That is crucial for the living organisms on the Earth because to much UV radiation will fry the living organisms in seconds.