Answer
See explanation.
Work Step by Step
When your arms are at your side, the force of gravity on the dumbbell exerts no torque about your shoulder joint.
However, if you hold your arm out horizontally, the weight of the dumbbell exerts a considerable torque on your shoulder joint, which you must compensate for to hold your arm steady. Your muscles must exert a large upward force on your upper arm, so you will tire more quickly.
