An anthropologist who considers the way in which a disease interacts with both human culture and the natural, non-human environment is considered a

The anthropologist described in the question is called medical anthropologist.

A medical anthropologist is someone who studies human health and disease as well as the healthcare systems and the biocultural adaption stemming from these conditions. They are concerned with community health programs in different ethnic groups, which also involve analyzing how different ethnic groups respond to disease prevention and management.

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."