Correct option is (2) red
Assuming that the right angled prism is an isoceles prism, so the other angles will be 45° each.
⇒ Each incident ray will make an angle of 45° with the normal at face PR
⇒ The wavelength corresponding to which the incidence angle is less than the critical angle, will pass through PR.
⇒ θc = critical angle
⇒ θc = sin-1(\(\frac 1\mu\))
⇒ If θc ≥ 45°
the light ray will pass
⇒ (θc)Red = sin-1(\(\frac{1}{127}\))
= 51.94°
Red will pass.
⇒ (θc)Green = sin-1(\(\frac{1}{1.42}\))
= 44.76°
Green will not pass
⇒ (θc)Blue = sin-1(\(\frac{1}{1.49}\)) = 42.15°
Blue will not pass
⇒ So only red will pass through PR.