Final answer:
The drug used for blocking H1 histamine receptors of C neurons is Diphenhydramine.
Step-by-step explanation:
The drug used for blocking H1 histamine receptors of C neurons is A) Diphenhydramine. Antihistamines are drugs that oppose the activity of histamine receptors in the body. H1-antihistamines bind to histamine H1 receptors in mast cells, smooth muscle, and endothelium in the body as well as in the brain.