Step 1: Check each verb against the noun "restraint." "Enforce restraint" and "maintain restraint" sound unnatural because these verbs pair with rules or discipline, not personal self-control. Step 2: "Remain restraint" is grammatically wrong since "remain" needs an adjective, not a noun. Step 3: The fixed expression "exercise restraint," meaning to hold oneself back, fits the sentence. \[\boxed{\text{Exercise}}\]