This answer choice doesn't give us any information we don't have. We know spears existed in the early Bronze Age. What we want is evidence for different methods of warfare in the late Bronze Age.

The suggestion here is that many prisoners of war were taken in the late Bronze Age. If anything, this might marginally weaken the argument that the late Bronze Age saw new methods of warfare designed to inflict higher casualties, or it could coexist with the argument. It doesn't strengthen the argument.

Evidence of warfare in the early Bronze Age doesn't help support the claim that new methods of warfare were developed in the late Bronze Age.

This doesn't support the claim that new methods of warfare were developed. It's possible armor and trophies of war were taken in the early Bronze Age, too, and the weapons could just be the same kind of spear.

This strengthens the argument. It suggests that one factor driving the higher casualty rate in the late Bronze Age was the use of arrows, a new kind of weapon that is different from the spears primarily used in the early Bronze Age.