What was unusual about the cotton club in harlem? a. african americans could not go there. c. its owners encouraged african-american performers. b. it catered to african-american audiences. d. only white musicians could perform there.

The correct option is this: AFRICAN AMERICAN COULD NOT GO THERE.
The Cotton Club was part of the night life in New York during the time the author wrote this book, that is, in the 1920s. The club was closed to African American individuals, who experienced great discrimination during that time.