Answer: Jenna's parents
Explanation: if Jenna's parents are throwing a party and either Jenna or one of her get drunk at the party, According to the social host liability it is Jenna's parents that will be liable because the law allow the person or group that host the party to be liable in some certain situation where a guest that was invited to the party got drunk or intoxicated and end up causing damages or injury to a third party.
Though in some states the social host liability law is only applicable to minors, state like Alabama, Arizoma, Florida and some few other states.