Take into account that the relation between speed and time can be written as follow:
where a is the acceleration of the motion. Furthermore, in a graph, acceleration a is the slope of the line. If the slope is positive, the vehicle accelerates, if the slope is negative, the vehicle decelerates.
Then, based on the given graph, you can notice that between points B and C, you have the greater negative slope. Then, on this interval the vehicle is decelerating the most.