The statement that is TRUE about the first modern gym that opened in the United States is: It was a YMCA.
The first YMCA in the United States, which opened in Boston in 1851, played a significant role in promoting physical fitness and was one of the early establishments to offer gym facilities. While there were various forms of physical exercise and training prior to this, the YMCA is often credited with being a pioneer in the development of organized physical fitness programs in America.