At gatsby's party, what seems to "start" the guests deciding it is time to leave? a) the bar runs out of alcohol. b) the orchestra finishes playing. c) couples begin to argue.

The answer is C.

From this excerpt taken from Chapter 3 of The Great Gatsby: "At two o’clock in the morning, as husbands and wives argue over whether to leave, a butler tells Jordan that Gatsby would like to see her.", we may conclude that this was the 'sign' that implies the moment from which the guests started to consider leaving the party, including the main characters at hand (Nick and Jordan), as their leaving is depicted in the outcome of the mentioned excerpt:

''Nick says goodbye to Gatsby, who goes inside to take a phone call from Philadelphia. Nick starts to walk home."