At point P,

$$\mu \sin 45^\circ = 1\sin r$$

$$ \Rightarrow \sin r = {\mu \over {\sqrt 2 }}$$

For light to come out of prism

sin r should be less than 1.

$$ \therefore $$ $$\sin r < 1$$

$$ \Rightarrow {\mu \over {\sqrt 2 }} < 1$$

$$ \Rightarrow \mu < \sqrt 2 $$

$$ \Rightarrow \mu < 1.414$$

$$ \therefore $$ The material which have refractive index less than 1.414, that material can emerge light out of the prism.

Here only red material have refractive index less than 1.414.

$$ \therefore $$ Red light will come out of the prism.