Correct Answer - A::B::C
It photon is emitted perpendicular to v, `f=f_(0)` because same part of energy of transition goes to recoil KE of the hydrogen as in the stationary atom. If photon is emitted in same direction as `v, f gt f_(0)` because now KE of the H atom decreases due to recoil. and it photon is emitted opposite to `v, f lt f_(0)` because KE of H atom increases due to recoil, the increase here is more than in a stationary atom .