Final answer:
Nick Carraway's statement "They're a rotten crowd" refers to Tom and Daisy Buchanan and Jordan Baker, highlighting the moral corruption he sees in the upper-class society depicted in F. Scott Fitzgerald's The Great Gatsby.
Step-by-step explanation:
When Nick Carraway remarks, “They’re a rotten crowd,” he is referring to the characters from F. Scott Fitzgerald's The Great Gatsby, including Tom and Daisy Buchanan, and Jordan Baker. These individuals are part of the wealthy, upper-class society that Nick interacts with, and he has come to see them as morally corrupt and careless, particularly following the accident and the events that ensue. They represent the decadence and the ethical decay of the time, highlighting Fitzgerald's theme that the quest for the American Dream often leads to moral decay, and that wealth and status do not equate to nobility or virtue.