The answer would be number two, which is "Jim bought Traci a CD for her birthday. The party was last Saturday night." Number one has a sentence fragment, which is Jim bought Traci a CD. Number three is a run-on. Hope I was able to help on time.
The sentence which is written correctly, without run-ons or fragments is the second one - Jim brought Traci a CD for her birthday. The party was last Saturday night. The first sentence ends abruptly and the last sentence is missing a full stop.
