Final answer:
Polysynaptic reflexes have longer synaptic delays, produce more complicated responses, and often include messages to the brain, making all of the given statements correct.
Step-by-step explanation:
Polysynaptic reflexes do indeed have longer synaptic delays than monosynaptic reflexes due to the involvement of multiple synapses. They produce more complicated responses than monosynaptic reflexes because they involve interneurons that connect sensory and motor neurons, which can modulate the strength and pattern of the response. Polysynaptic reflexes also often include messages sent to the brain, which means the brain can be aware of the reflex action and may influence the response.
Therefore, the correct answer to the student's question is d) All of these are correct. Synapses are indeed located in the gray matter of the spinal cord, and they facilitate the transmission of signals. Dendrites receive signals and can have dendritic spines, which increase synaptic connections, contrary to what choice b) of a given statement wrongly claims. Moreover, the sensory input can stimulate both short and long reflexes, depending on whether they project to the CNS or an autonomic ganglion.