The correct answer is option 'D', which means all of these components are present in the nervous system. Let's discuss each component in detail:

a) Brain:
- The brain is the main control center of the nervous system and is responsible for coordinating and controlling all bodily functions.
- It receives and processes sensory information, initiates voluntary movements, and regulates involuntary actions such as breathing and heart rate.
- The brain is divided into several regions, including the cerebral cortex, cerebellum, and brainstem, each with specific functions.

b) Spinal cord:
- The spinal cord is a long, thin, tubular bundle of nervous tissue that extends from the base of the brain to the lower back.
- It serves as a pathway for transmitting signals between the brain and the rest of the body.
- The spinal cord also plays a crucial role in reflex actions, which are rapid, involuntary responses to stimuli.

c) Cranial and spinal nerves:
- Cranial nerves are a set of 12 pairs of nerves that arise directly from the brain and primarily innervate the head and neck region. They control various functions such as vision, hearing, taste, and facial movements.
- Spinal nerves are a set of 31 pairs of nerves that arise from the spinal cord and innervate the rest of the body, including the limbs and trunk.
- These nerves carry sensory information from the body to the brain and transmit motor commands from the brain to the muscles and organs.

d) All of these:
- The nervous system is a complex network of organs, tissues, and cells that work together to process and transmit information throughout the body.
- It includes the brain, spinal cord, and peripheral nerves.
- The brain and spinal cord make up the central nervous system, while the cranial and spinal nerves form the peripheral nervous system.
- Together, these components allow us to sense our environment, control our movements, and regulate bodily functions.

In conclusion, the correct answer is option 'D' because all of these components (brain, spinal cord, and cranial and spinal nerves) are present in the nervous system. Each component has specific functions and plays a crucial role in the overall functioning of the nervous system.