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 ==> <u>speeding up</u>, or ==> <u>slowing down</u>, or ==> <u>turning</u>.
This graph is labeled "Velocity vs Time", but it isn't really. "Velocity" means <u>speed</u> AND <u>direction</u>. 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 <u>direction</u>, 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 .