Illinois Plumbing Code Practice

When a horizontal soil or waste branch connects to the soil or waste stack within two feet above or below an offset, a relief vent must be installed according to Section 890.1550(c) of the Illinois Plumbing Code. The relief vent helps prevent airlock situations and ensures proper venting of the drainage system when there is a change in direction in the plumbing layout due to the offset.

Options A, B, and D are not correct because they are not specifically required in this scenario as per the Illinois Plumbing Code.