Final answer:
The most likely weak muscles when the right iliac crest drops while standing on the left leg are the left hip abductors, particularly the gluteus medius, which is responsible for stabilizing the pelvis.
Step-by-step explanation:
When standing on the left leg, if the right iliac crest drops, it indicates a weakness in the muscles responsible for keeping the pelvis level when the opposite leg is raised. These muscles are the hip abductors on the supporting side, which in this case is the left side.
The primary hip abductor is the gluteus medius, which abducts and stabilizes the pelvis. If the left hip abductors were strong, they would contract to prevent the dropping of the contralateral (right) iliac crest. Thus, a weakness in this group would result in the observed inferior drop of the right iliac crest when lifting the right leg.