What is the smallest main vent that may be used on a 4-inch soil stack?

The smallest main vent that may be used on a 4-inch soil stack is 1 1/4 inches. This is based on plumbing codes and standards that dictate vent sizes corresponding to the diameter of the drainage piping. Specifically, for a 4-inch soil stack, the vent diameter must be sufficient to handle the ventilation needs of the system, allowing for proper air flow and the prevention of negative pressure that can impede drainage. A 1 1/4-inch vent provides the minimum cross-sectional area needed to support efficient venting for a system with a 4-inch soil stack, ensuring that it effectively prevents siphoning and allows for the escape of sewer gases.

The other options, although they are larger, are not the minimum required size for this scenario. The correct choice establishes a balance between adequate venting and compliance with regulations, ensuring that the system operates effectively without unnecessary oversizing.