Final answer:
The spinal cord is not part of the brain-testicular axis. So, the correct answer is option d.
Step-by-step explanation:
The correct answer is d. Spinal cord. The brain-testicular axis refers to the communication pathway between the hypothalamus, pituitary gland, and testes. The hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the pituitary gland to produce luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These hormones then stimulate the production of testosterone in the testes. The spinal cord, on the other hand, is not directly involved in this hormonal pathway.
So, the correct answer is option d.