The correct answer is D) size for threaded connections. The nominal pipe size of piping refers to the size designation used for the threaded connections, not the actual inside or outside diameter of the pipe. This is based on industry standards and regulations for pipe sizing, which are designed to ensure compatibility between different components in a hydraulic system. The nominal pipe size corresponds to the size of the threaded fittings, valves, and other components that the pipe will be connected to, rather than the physical dimensions of the pipe itself. The other options are incorrect because the actual inside diameter (A) and outside diameter (B) can vary depending on the pipe material and schedule, and the wall thickness (C) is a separate specification from the nominal pipe size.