When discussing water closet fittings, a 3 inch closet bend shall have a 4 inch by _______ inch flange.

The correct answer is 3 inches. A 3-inch closet bend typically requires a 4-inch by 3-inch flange because the flange must match the pipe diameter that it connects to while also fitting within the larger drain system. The flange is designed to secure the toilet and allow for proper drainage through the waste line.

Using a 4-inch by 3-inch flange allows a transition from the 3-inch pipe size of the closet bend to the standard larger 4-inch drain line. This fitting ensures that both the toilet and the drainage system operate efficiently without obstructions or leaks. Therefore, a 3-inch flange is necessary to maintain the correct size configuration and facilitate optimal function in plumbing systems.