What was the most common form of sacrifice in aztec society?
food offerings
the sacrifice of war captives
the sacrifice of slaves
voluntary bloodletting

The best answer is the sacrifice of war captives. Sacrificial victims were most often selected from captive warriors. Indeed warfare was often conducted for the sole purpose of furnishing candidates for sacrifice.

This was the so called flowery war where indecisive engagements were the result of the Aztecs being satisfied with taking only sufficient captives for sacrifice for sacrifice and where  the eastern Tlaxcala state was a favorite hunting ground.  

Those who had fought the best or were the most handsome were considered the best candidates.

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d.

the northwest territory became the most densely populated area in the u.s.