Final answer:
The correct answer is A. Spinal cord. It is a crucial part of the nervous system, extending from the brainstem to the pelvis, and acts as an information pathway between the brain and body.
Step-by-step explanation:
The cord of nervous tissue that extends from the brain lengthwise along the back in the spinal canal is known as the spinal cord. This structure is a long, thin, tubular bundle of nervous tissues that begins at the brain stem and extends down to the pelvis, passing through the spinal canal. It is encapsulated within the vertebral column, which provides protection, although the spinal cord itself is shorter than the vertebral column. The spinal cord plays a critical role as it acts like an information superhighway, relaying messages between the brain and the rest of the body, facilitating both sensory and motor functions.