B. Third class lever


When you swing a bat you, most likely grip it by the handle, turning this point into the fulcrum (the fulcrum is the point where a lever is placed to grip, or more clearly, the the axis of rotation, where it turns or is supported).

This particular characteristic makes it a third class lever, where the heavier part of the bat is the resistance, thus when swinged, the force at the end of the bat is reduced yet the speed is increased, in other words, the end of the bat moves faster than the grip, where the force is applied, increasing its momentum.