Which exercise would best increase upper-body strength?

The best exercise for increasing upper-body strength in this context is the dumbbell alternating curl. This exercise specifically targets the biceps, which are crucial muscles in the upper body. By performing alternating curls, the exercise isolates the biceps while also engaging the shoulders and stabilizing muscles of the upper body. This not only enhances muscle strength but promotes muscular endurance and coordination in the arms.

The other exercises, while beneficial for overall strength and conditioning, primarily target different muscle groups. For example, back squats are predominantly lower-body exercises that work the quads, hamstrings, and glutes. Lunges also focus on the lower body, engaging similar muscle groups as well as core stability. The seated row, although it does involve upper-body muscles by working the back and biceps, generally focuses more on strength in the back rather than isolating the arms in the same manner that the dumbbell alternating curl does. Thus, for a direct approach to increasing upper-body strength, especially in the arm muscles, the dumbbell alternating curl is the most effective choice.