At the center of the Prisoner's Dilemma is:

A) trust.
B) self-esteem.
C) fear of death.
D) ingroup bias.

Trust is at the center of the Prisoner's Dilemma, as the outcomes for each player rely heavily on whether or not they trust the other player to cooperate. Without trust, it is more likely that both players will choose to defect, resulting in a negative outcome for both. Self-esteem, fear of death, and ingroup bias are not central to the game theory of the Prisoner's Dilemma.