The model of human nature that sees behavior as the result of a complex system of wishes and emotions hidden by active defenses is the

The answer is psychodynamic model. Psychodynamic is a systemized study and theory of psychological forces that motivate human behavior, highlighting the inter play between unconscious and conscious motivation and the functions. This model is fundamentally grounded on Freud's psychoanalytic theory.

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."