The correct option is C v0/3
Since there is no external force acting on the 2 mass system, the momentum of the system and the velocity of the centre of mass remains constant.
Therefore, Vcm is the same before and after the collision, i.e;
Vcm,x=m1v1+m2v2m1+m2, as m2=2m1 & Vcm,y=0.
Vcm,x=m1vo+2m1×0m1+2m1=vo3.