Is it important for a government leader or planner to know the age of the population they are serving?

yes

Explanation:

yes

Explanation: this gives them and advatage

and gives them knowledge to know how many people they need to help in there  country

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."