A molecule . is made of more than one atom
is the same as a compound
must be made of more than one element
can only contain two atoms

I believe it's "Is made of more than one atom."

Explanation:

Can't be made of only 2 atoms, nor 1 element. Not all molecules are compounds, but some are. Make a choice.

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."