Final answer:
The train operator should hold the brake valve at position 12 to make a 15-pound brake pipe reduction using the ME-23 brake valve.
Step-by-step explanation:
To make a 15-pound brake pipe reduction using the ME-23 brake valve, the train operator should hold the brake valve at position 12. This position corresponds to a brake cylinder pressure of 60 psi and a brake pipe reduction of 15 pounds. Holding the brake valve at this position will release air from the brake pipe, causing the brake pipe pressure to decrease.