Final answer:
The correct answer is (a) Psoas, which is the muscle that binds the kidneys on the medial side. This muscle is a part of the posterior abdominal wall and helps in stabilizing the body.
Step-by-step explanation:
The kidneys are bound by the psoas muscles on the medial side, making option (a) Psoas the correct choice. The psoas major is part of the posterior abdominal wall, which also includes the quadratus lumborum muscle, lumbar vertebrae, and parts of the ilia of the hip bones.
This part of the core plays a key role in stabilizing the body and maintaining posture. The other muscles listed, such as the quadratus lumborum, transversus abdominis, and the external oblique, are also important components of the abdominal and posterior abdominal walls but are not the correct choices for what binds the kidneys medially.