a sound wave strikes the surface of a reflecting body perpendicularly.the angle of incidence is:

90 45 0 none of these

it should be 45 right?

The answer will be $\theta =0^o$.

It is because, the angle of incidence is the angle between the normal and the angle at which the light/sound wave is incident on the surface. So, as the question says that the sound wave is incident perpendicularly on the reflecting surface then there the angle of incidence will be zero degree.