Final answer:
Strict adherence to classical structures is not a defining characteristic of contemporary theatre in the United States; modern theatre often resists these traditions to embrace diversity, experimentation, and engagement with social issues.
Step-by-step explanation:
The question seeks to identify which option is not a major characteristic of contemporary theatre in the United States. The given options are diversity in storytelling, experimentation with form, strict adherence to classical structures, and exploration of social issues. Contemporary US theatre is well known for its diversity in storytelling, with writers and performers from a multitude of backgrounds who bring their unique perspectives to the stage. It is also known for experimentation with form, as artists often challenge traditional structures to tell stories in new and engaging ways. Furthermore, exploration of social issues has been a central aspect of modern plays, as theatre practitioners use the medium to critically engage with contemporary societal problems and to provide a voice for marginalized groups.
However, strict adherence to classical structures is not a defining characteristic of contemporary theatre in the United States. Modern theatre tends to resist the strict formulas and confines of ancient Greek and classical European theatre that have been historically upheld by Western tradition, choosing instead to embrace flexibility and innovation in its approach.