The best measure for comparing a country’s aggregate output over time is a) Nominal GDP.
b) Real GDP.
c) Nominal GDP per capita.
d) Real GDP per capita.
e) Average GDP per capita.

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."