Final answer:
The part of the descending analgesic pathway from the options provided is the reticulospinal tract. It is involved in pain modulation, while other options like the spinothalamic tract are ascending pathways for sensory input.
Step-by-step explanation:
The correct answer to which of these tracts is part of the descending analgesic pathway is d. reticulospinal tract. This tract is involved in the inhibition of pain sensation as part of the descending analgesic (pain control) pathways. The spinothalamic tract and the dorsal column system are major ascending sensory pathways in the spinal cord that connect the periphery with the brain, and they convey sensations such as pain, temperature, vibration, fine touch, and proprioception to the brain.
On the other hand, the reticulospinal tract is one of the extrapyramidal tracts, such as the vestibulospinal and tectospinal tracts, that modulate motor functions, including pain inhibition.