The frith family consists of a mother, a father, and their biological children. sociologists classify this as a(n) family.

The firth family consists of a mother, a father, and their biological children. Sociologists classify this as a nuclear family.

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."