A 45-year-old storeman has three weeks of neck pain radiating into the right arm. Pushing a heavy door open with that arm is weak, and so is bending the wrist forward. The triceps jerk is absent on the right; the biceps and brachioradialis jerks are equal and normal. Sensation over the thumb is normal but reduced over the middle finger. Finger spreading and grip are strong and the small hand muscles look normal.
Compression of which one of the following nerve roots best explains this storeman's arm findings?