Final answer:
The gracilis muscle does not insert onto the femur but rather on the tibia and is involved in thigh adduction and knee flexion. All other muscles listed insert onto the femur. The correct answer is option 4.
Step-by-step explanation:
The question pertains to the anatomy of muscles that insert onto the femur. Among the options given, the pectineus, adductor longus, and adductor brevis all have insertions on the femur. The pectineus adducts and flexes the femur at the hip and is found in the femoral triangle. The adductor longus and adductor brevis are also involved in the adduction and medial rotation of the thigh, with the adductor longus also assisting in flexing the thigh, and the adductor brevis playing a role in medially rotating the thigh.
However, the muscle known as the gracilis does not insert onto the femur; instead, it inserts onto the tibia. The gracilis is involved in adducting the thigh and flexing the leg at the knee. Therefore, the correct answer to the question, "Which of the following do not insert onto the femur?" is the gracilis.