Before the string breaks, the velocity of the ball is constant and positive (since it's being pulled in the upward direction). This eliminates option C.
When the string breaks, the ball is now in freefall and accelerating downward due to gravity, which means its velocity will be decreasing (or more precisely, growing larger in the negative direction). This eliminates option A.
Also when the string breaks, the ball still has an instantaneous upward velocity equal to the constant velocity it had before the string broke. So when it's released, the ball still travels slightly upward before slowing down and accelerating downward. This means the graph of its velocity should be continuous, which eliminates option B.