The statement that is TRUE about group exercise is:
It gained popularity in the 1980s with the aerobics craze.
Group exercise classes, especially those focused on aerobic workouts, became widely popular during this time, leading to the establishment of many fitness studios and classes. The other statements are not accurate. Group exercise can be as effective, if not more so, than exercising alone for many individuals, and it's generally motivated by positive reinforcement rather than negative. Additionally, group exercise was not created by Otto Kohler in the 1920s.