Correct Answer is: A) Decreases
As the light enters from rarer medium to the denser medium, the speed of light decreases but the frequency remains the same.
The relation between Speed of light, wavelength and frequency is given as:
V = nλ
Where V is speed
N = frequency of light
λ = wave length
n = v/λ
In denser medium, speed decreases, therefore wavelength also decreases. To keep frequency constant.