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in elastic collisions kinetic energy is always conserved
so, the sum of velocity b4 collision must equal that after collision
that means
u1+v1=u2+v2
but at the same time you need to keep a reference direction
lets say towards the right is positive
so the eqn for this case will be ux+(-vx)=(-uy) + vy
now you can simply rearrange to get the correct answer that is A
hope its clear now