For a 20-year-old, the best source of active bone marrow would be located in the:

In adults, the iliac crest is the most reliable and accessible source of active bone marrow. It contains red bone marrow, which is responsible for hematopoiesis, the process of producing blood cells. The iliac crest is preferred for procedures like bone marrow aspiration or biopsy because it is easily palpable and has a significant volume of accessible marrow.

While the femur, distal radius, and tibia do contain active bone marrow, their use is less common. The femur, particularly the proximal end, does have red marrow, but it's not as accessible for procedures as the iliac crest. The distal radius generally has less active marrow in adults compared to the iliac crest, and the tibia, while it may still contain red marrow, is typically not sought after for marrow sampling due to its smaller volume and deeper location. Therefore, the iliac crest stands out as the best choice for obtaining active bone marrow in a young adult.