Final answer:
The size of the roof drain according to the 2015 IPC should be based on the manufacturer's published flow rate, which is vital for adequate drainage capacity to prevent water accumulation.
Step-by-step explanation:
The 2015 International Plumbing Code (IPC) stipulates that the size of the roof drain must be determined based on the manufacturer's published flow rate. Thus, the correct answer to the student's question is 'a. Flow rate'. The flow rate is indicative of how much water the drain can handle and is essential for ensuring the roof drainage system is adequate for preventing water accumulation and possible damage. Manufacturer specifications provide data including the maximum flow rate that a roof drain can effectively handle, which is based on factors such as the design of the drain, the dimensions, and the materials used in its construction.