Although an exact count is not available, it is likely that the human brain has as many as
a) 10,000 neurons.
b) 500,000 neurons.
c) 1 million neurons.
d) 10 million neurons.
e) 100 billion neurons.

which sentences ? that question is incomplete

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."