The main idea of the paragraph is that jumping rope is a very good exercise (a). The paragraph highlights the benefits of jumping rope, such as its popularity among various training programs, its accessibility, and its effectiveness in improving heart health and sports performance. The paragraph emphasizes that jumping rope is a convenient, inexpensive, and enjoyable exercise that can benefit almost anyone. While other options are briefly mentioned, such as making or finding a jump rope, the main focus is on the exercise itself and its positive impact on health and athletic abilities.