Final answer:
Nick notices a change in the atmosphere at Gatsby's party when Tom and Daisy attend, as tension and societal divides become apparent. Gatsby refers to Tom as "the polo player" to subtly highlight social status and class distinctions.
Step-by-step explanation:
The atmosphere at Gatsby's party changes for Nick when Tom and Daisy attend because their presence introduces tension and the reality of societal and class distinctions. Gatsby's deliberate reference to Tom as "the polo player" all evening is a subtle way of highlighting Tom's status, wealth, and the elite sportsmanship associated with the upper class.
It underscores how Tom belongs to the old money demographic, a group that Gatsby, despite his wealth, cannot comfortably infiltrate. This distinction illustrates the underlying conflicts in F. Scott Fitzgerald's novel, where characters such as Gatsby attempt to break into a tightly knit and deeply judgmental social circle that resists newcomers, regardless of their wealth.