Final answer:
The correct answer is nerves. Nerves transmit pain signals from the knee to the central nervous system.
Step-by-step explanation:
The correct answer is a) Nerves. When you scrape your knee, pain is relayed from your knee to your central nervous system by nerves. Nerves are specialized cells that transmit signals between different parts of the body, allowing for the communication of sensory information, such as pain, to the brain.