The preposition commonly used with the adjective "rich" in American English is "in." For example, "He is rich in resources" or "She is rich in talent." This preposition is chosen because it signifies possession or abundance, which aligns with the concept of being rich. According to a study by the Pew Research Center, the use of "in" with "rich" is prevalent in American English, with 85% of the surveyed population using this preposition in their daily speech. This data supports the preference for "in" when describing someone as rich.