Acceleration is a measure of how fast the velocity of a particle/object is changing at a point. Therefore, acceleration can be measured by finding the slope of the velocity of the particle at a certain point.
Since we are given the graph of velocity, we have to find the slope of the particle at point B. In the given graph, the slope of the velocity of the particle at point B is zero. Therefore, the acceleration of the particle at point B is 0.
Hope this helps!