Medulla is the part of spinal cord which forms a connection between brain and spinal cord. From medulla spinal cord starts arising. Other part like cerebrum is for thinking activity and is no way related to arising of spinal cord. Cerebellum controls body posture and is also not related to spinal cord. Pons is above the medulla. Only medulla is attached with the spinal cord and this is depicted from the following diagram:
Therefore, correct option is (b) Medulla.