Whenever one turns the pages of a book, uses a pen to take notes, or raises one's hand, one's action is possible due to the nervous system.

Somatic nervous system

Explanation:

The somatic nervous system is the component of the peripheral nervous system concerned with voluntary skeletal muscular regulation of body movements. The somatic nervous system is made up of afferent nerves or sensory nerves, as well as efferent nerves or motor nerves.

it continues in business

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."