All living organisms are composed of A.
only one cell.
B.
one or more cells.
C.
at least three cells.
D.
at least 100 cells.

B. one or more cells

Explanation:

organisms message of one cell are called unicellular, for example- amoeba, euglena, paramecium, etc. and organisms made up of 2 or more cells are called multicellular, for example- animals, human beings, etc.

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."