What supports the plumb bobs when plumbing the door frame?

When plumbing a door frame, plumb bobs are typically supported using a method that allows for precise vertical alignment. The correct answer involves using magnets to hold the plumb bobs in place. Magnets provide stability and ensure that the bob remains in a fixed position, allowing for accurate readings to establish a vertical line. This is especially useful in situations where a level surface is essential, as it minimizes any potential errors caused by movement or vibrations.

While chains, ropes, and clamps may be used in various construction or alignment tasks, they do not provide the same level of precision or stability as magnets for this specific application. Chains and ropes can introduce variability and are less effective at holding the plumb bob firmly in one place. Clamps might restrict movement but wouldn’t effectively support a plumb bob in a way that allows for the necessary adjustments in plumbing the door frame. Therefore, using magnets is the most reliable method for ensuring an accurate alignment during the installation process.