Which muscles are used to flex the knee joint?

The soleus crosses only the ankle joint, not the knee, so it cannot contribute to knee flexion despite the gastrocnemius being a valid knee flexor.

The hamstrings (biceps femoris, semitendinosus, semimembranosus), gracilis, and sartorius all cross the posterior knee joint and insert below the knee, making them primary knee flexors. The gracilis runs from the pubis to the medial tibia, and the sartorius runs from the anterior superior iliac spine to the medial tibia, both contributing to knee flexion. This combination correctly identifies muscles from multiple origin points that share the function of flexing the knee.

The rectus femoris, vastus lateralis, and vastus medialis are all quadriceps muscles that produce knee extension, not flexion.

The vastus lateralis is a quadriceps muscle responsible for knee extension, not flexion, making this combination anatomically incorrect for knee flexion.