What belongs in section 1?

A presidential system, or single executive system, is a form of government in which a head of government (president) leads an executive branch that is separate from the legislative branch in systems that use separation of powers. This head of government is in most cases also the head of state.

So, seeing as the legislative and executive branches are separate, the answer  is D

powerful warriors rose to become chiefs, who had a privileged place in society.

after the spaniards introduced horses to plains native americans, the lives of the tribes changed. tribes became less settled and sedentary. the horse allowed them to hunt animals such as bison or buffalo, and, instead of staying in one place, tribes began to be nomadic.