Final Answer:
According to the IPC, if a 4" horizontal branch of 160 DFUs discharged into the top branch interval of a stack, the stack would have to be d. Sized in accordance with the applicable plumbing regulations.
Step-by-step explanation:
In plumbing, the International Plumbing Code (IPC) provides guidelines for the design and installation of plumbing systems. If a 4" horizontal branch with 160 Drainage Fixture Units (DFUs) discharges into the top branch interval of a stack, the stack must be sized in accordance with the applicable plumbing regulations (Option D).
This means that the size of the stack should meet the code requirements to handle the increased load of DFUs. Options A, B, and C are not specifically mentioned in the context of this scenario and do not align with typical responses to a horizontal branch discharging into a stack in plumbing design.
Option D is the answer.