What is the minimum slope required per foot toward the tank for vapor and vent lines?

The minimum slope required for vapor and vent lines toward the tank is indeed 1/6. This slope is necessary to ensure proper drainage of any condensate that may accumulate in the lines. Having this appropriate slope prevents liquid buildup, which could otherwise obstruct the flow of vapors and potentially lead to hazardous conditions.

In the context of installation and retrofitting, maintaining this slope is crucial for the functionality and safety of the system. A slope that is not steep enough may allow vapors to condense and lead to liquid accumulation, which can cause pressure build-up in the vent lines and compromise the operation of the storage tank and related systems.

The other options represent slopes that are either too shallow or nonsensical for the context, and therefore do not meet the necessary requirements for effective vapor and vent line performance. Choosing a slope like 1/18, for instance, would not ensure adequate drainage, which is vital for maintaining safety and compliance with regulations.