When holding a dumbbell to bend the elbow upward, if the forearm is regarded as a lever, the fulcrum is at the B position of the elbow joint, the biceps brachii contracts and lifts the forearm as the power, and the force exerted by the dumbbell on the hand is the resistance. In the process of bending the dumbbell upward, the power arm is smaller than the resistance arm and belongs to a laborious lever.

So choose D.