Answer
The wavelength of the light in water is $0.375~\mu m$
Work Step by Step
We can find the index of refraction in water:
$v_{water} = \frac{c}{n}$
$n = \frac{c}{v_{water}}$
$n = \frac{c}{0.750~c}$
$n = 1.333$
We can find the wavelength in water:
$\lambda_{water} = \frac{\lambda_{air}}{n}$
$\lambda_{water} = \frac{0.500~\mu m}{1.333}$
$\lambda_{water} = 0.375~\mu m$
The wavelength of the light in water is $0.375~\mu m$.
