The route by which a drug is introduced into the subarachnoid space surrounding the brain and the spinal cord is:

This route involves the administration of drugs into the peritoneal cavity, which is not connected to the subarachnoid space. Intraperitoneal injections are primarily used for delivering medications to the abdominal organs or for chemotherapy, making it unsuitable for targeting the central nervous system.

Intravenous administration delivers drugs directly into the bloodstream via a vein. While this route provides rapid systemic effects, it does not specifically target the subarachnoid space surrounding the brain and spinal cord, which is essential for central nervous system treatment.

Intratracheal administration involves delivering drugs directly into the trachea, primarily for respiratory conditions. This method does not reach the subarachnoid space and is not applicable for treatments aimed at the brain or spinal cord.