(a) Gamma rays are used for the treatment of certain forms of cancer. Their frequency range is $3 \times 10^{19}$ $\text{Hz}$ to $5 \times 10^{20}\,\text{Hz}$. (b) The thin ozone layer on top of stratosphere absorb most of the harmful ultraviolet rays coming from the Sun towards the Earth. They include UVA, UVB and UVC radiations, which can destroy the life system on the Earth. Hence, this layer is crucial for human survival. (c) The momentum transported by electromagnetic waves is given by $p=\;\frac{U}{c}=\;\frac{h v}{c}$ where $U$ is the energy transported by electromagnetic waves in a given time and $c$ is speed of electromagnetic waves in free space. Now, $h=6.62 \times 10^{-34}\,\text{J}\,\text{s}, c=3 \times 10^{8}\,\text{m}\,\text{s}^{-1}$ Therefore, even for $\gamma$-rays $(v \approx 10^{20}\,\text{Hz})$, $p\;=\;\frac{6.62 \times 10^{-34} \times 10^{20}}{3 \times 10^{8}}$$\;=2.2 \times 10^{-22}\,\text{kg}\,\text{m}\,\text{s}^{-1}$Thus, the amount of the momentum transferred by the e.m. waves incident on a surface is very small.