What does the ozone layer primarily block?

The ozone layer primarily functions as a protective shield in the Earth's stratosphere by absorbing the majority of the sun's harmful ultraviolet (UV) radiation. Specifically, it effectively blocks all UVC rays, which are the most hazardous to living organisms, and certain wavelengths of UVB rays.

UVB rays, while partially blocked by the ozone layer, are not completely eliminated from reaching the Earth's surface; thus, some UVB rays can still contribute to sunburn and skin damage. The UVC rays, however, are almost entirely absorbed by the ozone layer and do not reach the surface, making the ozone layer crucial for protecting life on Earth from these particularly damaging wavelengths of radiation.

So, the correct answer highlights the critical role of the ozone layer in absorbing harmful UV radiation, ensuring that only a fraction of UVB rays, along with UVA rays, make it to the surface.