There's not enough information in the graph to determine whether it was or it wasn't. It could have been, but we can't tell.
Before I explain this answer, it's VERY important to know exactly what "accelerating" means:
-- "Accelerating" does NOT always mean "speeding up".
-- "Accelerating" means ANY change in velocity.
-- That can be ==> speeding up, or ==> slowing down, or ==> turning.
This graph is labeled "Velocity vs Time", but it isn't really. "Velocity" means speed AND direction. The graph only shows us the object's speed, but nothing about its direction.
During time-interval-B, the speed isn't changing, but the object could still be changing direction, like turning a corner, or on a curved path. We don't know. The graph doesn't show us that information.
The object could be accelerating during time-interval-B. It's true that the speed wasn't changing. But maybe the direction was. If the direction was changing, then we have to say the object was 'accelerating' during the interval labeled B .
