The cumbersome spears that were the principal weapons used by certain tribes in the early Bronze Age precluded widespread casualties during intertribal conflicts.
Claim (hence)
By the late Bronze Age, these tribes had developed new methods of warfare designed to inflict many casualties.
Intermediate Conclusion (indicates that)
In late Bronze Age, wars between tribes were frequent / casualty rate was high.
Evidence
In early Bronze Age, the main weapons were cumbersome spears. This prevented widespread casualties. In late Bronze Age, there was a much higher number of warrior tombs.
Evaluate
The author is interpreting the significance of a Curious Comparison: why were there way more warrior tombs in late Bronze Age?
Her explanation: wars were frequent, the casualty rate was high, and tribes had developed new methods of warfare designed to inflict many casualties.
Goal
To strengthen a Curious Fact / Explanation argument, we primarily search for an answer that bolsters the plausibility of the Author's Story.
In this case, the question stem is specifically assigning us to support the plausibility of the claim that their were .
(There's a small chance, though, that the correct answer could rule out an Alternate Explanation for why there is a higher number of warrior tombs.)