The diameter of a relief vent serving stacks of more than 10-branch intervals shall be AT LEAST

This option suggests a proportional sizing based on the connected branch's diameter, which may lead to insufficient venting capacity. Such a variable size could compromise the effectiveness of the vent, especially in systems with many branches where adequate airflow is critical.

While 1 1/2 inches could be adequate for smaller venting scenarios, it does not meet the minimum requirement for systems with more than 10-branch intervals. This size fails to provide the necessary ventilation to handle the increased demands of larger plumbing systems, thus potentially leading to drainage issues.

This choice suggests that the relief vent should match the vent stack diameter, which may not be sufficient for larger systems. The code specifies a minimum of 2 inches specifically to ensure that the venting system can handle the airflow requirements effectively, which may not be guaranteed by merely matching the stack's size.