Final answer:
Muscle spindles detect the stretching of muscles, and sensory neurons transmit this information to the brain and spinal cord. The correct answer to the question is 2) Sensory neurons.
Step-by-step explanation:
The structures that detect changes in the length of muscle fiber and signal through afferent neurons to the spinal cord and the brain are called muscle spindles. The muscle spindle is a stretch receptor that senses how much a muscle is being stretched or lengthened. When a change is detected, this information is sent to the central nervous system via sensory neurons, which are part of the afferent pathway.
Therefore, the correct answer to the question is 2) Sensory neurons. These neurons are responsible for transmitting sensory information from the muscle spindles to the spinal cord and brain, allowing the body to maintain balance and posture and coordinate movements.