I want explanation with the correct answer as to why the answer is 0 degree despite refraction of light .

Refraction only occurs when the light is incident to the surface at some positive angle. When light is perpendicular to a surface there is no bend to the ray.

Using Snell's law, the math looks like this

n₁sinθ₁ = n₂sinθ₂

at an incidence of 0°

n₁sin(0) = n₂sinθ₂

         0 = n₂sinθ₂

as n₂ is always a positive number, sinθ₂ must equal 0 meaning θ₂ = 0

So while there will be refraction of light a position O, which is the exact center of the semicircular prism and the refracted ray will follow a radial line, there will be no refraction of light at P because the light is traveling perpendicular to the surface there.  There will be a change in light velocity because of the change in the medium, but no refraction.

Explanation: