The correct option is A
Momentum = mass$$\times$$velocity.
The Law of conservation of momentum says that whenever two or more bodies collide, and there is no external force, then their total momentum before collision is equal to that after collision.
Since one disc is moving towards right, its momentum is $$2Mv_0$$
Since 2 discs are moving to left, their momentum is $$-2Mv_0$$
Thus total momentum before collision is $$0.$$
By the Law, after collision should also be $$0.$$
Since the velocity of disc $$m$$ is $$0$$, its momentum is also 0.
For the total momentum to be zero, the velocity of the other two discs should also be zero.
Hence speed of M after collision = 0.