The main reason we may not notice the young-old is that: a. they are a problem-prone minority. b. there are relatively few of them. c. they are not integrated into the community. d. they do not fit our stereotypes of the elderly.

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."