The size of the house drain shall be determined by the total number of ______ connecting to the house drain.

The size of the house drain is determined by the total number of fixture units connecting to it. Fixture units are a standardized measure used to assess the potential demand on a plumbing system from various fixtures, such as sinks, toilets, and showers. By evaluating the fixture units, plumbers can ensure that the drain is appropriately sized to handle the wastewater flow from these fixtures without risking clogs or overflows.

When calculating the size of the drain based on fixture units, it provides a comprehensive understanding of the total load on the system, accounting for the variability in usage and the specific contributing fixtures. This measurement is essential in designing a plumbing system that operates efficiently and meets code requirements, ensuring that the drainage system can handle peak usage.

Other options, while relevant to plumbing design, do not provide the same standardized basis for determining drain size as fixture units do. This differentiation makes fixture units the correct and most effective measure for sizing house drains.