What method shall be permitted for reducing the size of openings of fittings when standard sizes are unavailable?

The correct option is the hexagonal (face bushing) because it is specifically designed to reduce the size of fittings through its unique geometric shape, which allows for an effective transition between different pipe sizes. This method provides a reliable and efficient way to connect pipes of varying diameters, ensuring a secure and leak-free joint.

In the context of sprinkler systems, using a face bushing can accommodate fittings when standard sizes are not available. It is preferred in many installations due to its design that allows for greater versatility and adaptability in piping systems.

Conical bushings and cylindrical reducers serve similar purposes but may not be as commonly preferred as face bushings for certain applications in sprinkler systems. A square plate does not serve this function and is not relevant for reducing pipe sizes. Therefore, selecting the hexagonal (face bushing) aligns with the best practices outlined in NFPA 13 for ensuring proper system installation and efficiency.