Final answer:
Polyamory is the atypical relationship style among young adults in the United States, as it involves multiple consensual intimate relationships, unlike the more conventional two-person unions of monogamy and cohabitation.
Step-by-step explanation:
The atypical relationship style of young adults in the United States mentioned in the choices is polyamory. Unlike monogamous marriage, serial monogamy, or cohabitation, all of which involve two-person unions, polyamory refers to having intimate relationships with more than one partner, with the informed consent of all partners involved. While monogamous marriage and serial monogamy are commonplace, and cohabitation has become increasingly accepted in U.S. society, polyamory still remains less conventional.