Final answer:
The loaded left pelvis experiences medial rotation (option C) in the horizontal plane when the right leg takes an unloaded step forward. This is facilitated by the medial rotators of the thigh, which adduct and rotate the femur toward the midline of the body.
Step-by-step explanation:
When taking a step forward with the right leg while unloaded, the position of the loaded left pelvis in the horizontal plane would be medial rotation. As the right leg moves forward, the left hip bone remains stable, and the muscles around the hip joint control the movement. According to the anatomical terms of movement, medial rotation is the movement that brings the anterior surface of the limb toward the midline of the body. This motion is facilitated by the internal (medial) rotators of the thigh such as the adductor longus, adductor brevis, and adductor magnus, which adduct and medially rotate the femur at the hip.
Compared to when the right leg moves backward, which would involve lateral rotation to maintain balance and stability, stepping forward with the right leg causes a relative medial rotation of the left pelvis. This is in line with the role of the pelvic girdle in transferring body weight to the lower limbs and facilitating stable movement, as it provides a fixed center of gravity and allows the muscles to perform movements with efficiency and less energy expenditure.