What kind of market structure is mars inc.?

Oligopoly

An oligopoly is a market structure in which a few organizations dominate the entire industry. While a market is shared within a few organizations, it is assumed to be extremely intense. Although only a few firms are controlling the market, it is potential that several small firms may additionally function in the market.

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."