Blane deals with people who have suffered some sort of brain damage, them try to use different areas of the brain that perform functions that were previously performed by the areas that are now damaged. blane's specialty would be best described as

Blane is a neurologist.

This specialist explores the nervous system, verifies how muscle strength works, reflexes, sensitivities, balance, among other disorders that affect the nervous system, in order to find the cause and the correct treatment for each patient, sometimes it works hand in hand with the psychologist to diagnose mental problems.

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."