What Is the "Dilemma" in the Prisoner's Dilemma Game

What is the "dilemma" in the prisoner's dilemma game?

A) Individuals cannot choose the proper strategy because the payoffs constantly change.
B) All strategies provide equal payoffs regardless of what one's partner does.
C) Cooperation can never evolve despite the fact that individuals always do best when cooperating.
D) The payoff for cooperating is substantially higher than the payoff for cheating unless both individuals in a pair opt to cheat.
E) It pays each individual to cheat every time, but mutual defection is more costly than mutual cooperation.