$\textit{a.}$, Refraction takes place because, when light enters a new medium (with different density) it bends, since it travels with different speeds in different mediums.

$\textit{b.}$, There have to be two optically different mediums, since it occurs when ray of light travels from one medium to another, optically different one.

$\textit{c.}$, Light travels slower through the water than through the air:

$c_{air} = 3 \cdot 10^{8}\, \mathrm{m/s}$

$c_{water} = 2.25 \cdot 10^{8}\, \mathrm{m/s}$

a. Refraction is the bending of light when it enters a different medium from another medium. This is because light travels at different speeds in different mediums.