Answer: to confess.
Explanation:
Game theory is used to determine the optimal course of action for rational individuals.
Prisoners dilemma is a form game theory that seeks to find the optimal choice for prisoners between confessing and not confessing.
The optimal choice for the prisoner in this question is to confess because if he does, he spends 6 years in jail or 2 years in jail which is better to not confessing and spending either 4 yards or 10 years in jail.