The principle says that when consent to an agreement is not free, such as consent obtained under threat, the agreement can still be enforced, but only if the party whose consent was not free chooses to enforce it. Here, A obtained B's consent by threatening B's girlfriend at gunpoint, so B's consent was clearly not free. Let's check each option against this principle.
Because B's consent was obtained under threat, the principle places the choice to enforce the agreement in B's hands alone.
Therefore, the correct answer is B can enforce the agreement.
The principle sets out a conditional rule: if a party's consent to an agreement was not free, then the agreement is enforceable only at the option of that specific party, and the option belongs to no one else. The facts show A obtained B's consent by threatening B's girlfriend at gunpoint, which means it is B's consent, not A's, that was not free. Let's test each option against this rule.
Because B is the party whose consent was obtained through threat, the rule places the enforcement option in B's hands alone, and no other outcome fits its wording.
Therefore, the correct answer is B can enforce the agreement.