Ahacker subculture that probes computer systems, usually without permission, to find security flaws as an intellectual exercise or public service sometimes call themselves:

carefully examine the map above. based on your knowledge of colonialism in africa, the area attributed to belgium on this map is most likely the democratic republic of the congo.

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."