Hence some archaeologists claim that by the late Bronze Age, these tribes had developed new methods of warfare
designed to inflict many casualties.
(D) Some tombs of warriors dating to the late Bronze Age contain armor and weapons that anthropologists believe were trophies taken from enemies in battle.
If you look at the two bold phrases, the stem's conclusion is the newer weapons were developed, but D implies that whatever weapons and armor were found were from enemies and therefore not developed by the tribes themselves.
This is not as strong as saying that "arrowheads", which consitutes newer weapns, were found in the tombs.
Hope this helps.
Was D out coz it does not prove the casualties.. Mere possession of weapons do not prove casualties..