According to Coast Guard Regulations (46 CFR), which of the following is permitted in boiler fuel oil service system discharge piping?

Correct Answer: Option D — Screwed bonnet valves of the union bonnet type.

Under 46 CFR, fuel oil service discharge piping handling high pressures and flammable fluids prohibits standard pipe unions, street ells, and bushings to prevent joint failure. Screwed bonnet valves of the union bonnet type are permitted because their robust mechanical design withstands high-pressure thermal and mechanical stresses.