Correct Answer - A
Let angle between initial velocities be `pi-theta` and the situation is an showin in figure. Conserve the momentum along `x`- and `y`-direction.
`mv-mvcostheta-2mv/2cosalpha`, for `X`-axis
`mv sintheta=2mv/2sinalpha` for `y`-axis
Solving above equation `theta=60^(@)`
Required angle `=pi-60^(@)=120^(@)`