The author of Document D mentions "a 1925 Army War College study” in line 4
primarily in order to

to force terrorists groups to stop attacking western targets using massive military force.

the correct answer is "ethnic heritage".

a person's ethnicity can influence a lot the way he/she votes.

in the united states, for example, black people tend to vote democratic. it happens because the society builds a strong division among people, and equals tend to mingle and support equals. black people's votes in a whole will most certaintly lean towards a candidate or a party which supports the fight against racism.