The term began to be used during the Iraq War and described the forces opposed to American involvement. Members are typically organized into covert groups that engage in organized campaigns of extreme violence.

The term terrorists/revolutionaries began to be used during the Iraq War and described the forces opposed to American involvement. Members are typically organized into covert groups that engage in organized campaigns of extreme violence.

d. i assume the underlined word is "role." reformat if it isn't.

explanation:

role is closest to part because when replacing words, "playing a vital decision," "playing a vital activity," and "playing a vital dollar" don't make logical sense. the only meaningful choice is "playing a vital part."

this fits one of merriam-webster's definitions of role: "a function or part performed especially in a particular operation or process."