Final answer:
a. pectineus The pectineus muscle causes medial rotation of the thigh, adducts, and flexes the femur at the hip.
Step-by-step explanation:
The contraction of the muscle that causes medial rotation of the thigh is the pectineus. This muscle, which is found in the femoral triangle of the hip and leg junction, not only adducts but also flexes the femur at the hip joint. The other muscles mentioned, such as the obturator externus and gracilis, have different primary actions. The obturator externus is known for laterally rotating the femur at the hip, while the gracilis adducts the thigh and flexes the leg at the knee.
The pectineus muscle is responsible for the medial rotation of the thigh. Along with the adductor longus, adductor brevis, and adductor magnus, the pectineus adducts and flexes the femur at the hip.