Question

which type of uv radiation is the least energetic, and thus, least harmful? uv - d uv - a

Explanation:

Among common types of UV - radiation (UV - A, UV - B, UV - C), UV - A has the longest wavelength and lowest energy. There is no UV - D in the common classification of UV radiation. Longer wavelength corresponds to lower energy according to the electromagnetic - radiation relationship $E=\frac{hc}{\lambda}$ (where $E$ is energy, $h$ is Planck's constant, $c$ is the speed of light, and $\lambda$ is wavelength).

Answer:

UV - A