Final answer:
Option (4), Nick discovers that Tom has a mistress, revealing Tom's decadent and irresponsible lifestyle.
Step-by-step explanation:
At dinner, Nick finds out that Tom has a mistress. The context provided suggests indulgence in a decadent lifestyle, illustrating Tom's disregard for his relationships and responsibilities. Notably, in different scenes provided, Tom abandons his pregnant fiancé Sarah Young and marries for money, highlighting his constant search for pleasure and wealth at the cost of his morals and those who care about him.
His affairs and lifestyle choices are a critique of the lack of moral fortitude and the consequences of living a life consumed by greed and desire.