How should the surfaces on which flexible cords bear be designed according to the outlet box guidelines?

The correct choice emphasizes that surfaces bearing flexible cords should be flat and smooth to ensure proper functioning and longevity of the cords. A flat and smooth surface reduces the risk of abrasion and wear on the flexible cords, which could otherwise lead to damage or electrical issues over time. This design consideration is crucial for avoiding mechanical stress on the cords, which can arise from sharp edges or uneven surfaces that might trap the cords in a position where they are subject to excessive bending or pinching. By maintaining flat and smooth surfaces, you help to reduce strain, enhance safety, and contribute to reliable electrical performance in installations.