If a conduit is missing a pull line, what can be used as a substitute?

The use of an unused cable already installed in the conduit as a substitute for a missing pull line is a practical solution. This method leverages resources already present within the conduit to assist in future installations or cable pulls. An existing cable can provide the necessary guidance and support for dragging new cables through the conduit without requiring additional materials.

Using an unused cable allows for a smoother operation since the cable is designed to withstand the pulling forces and it can help navigate bends and turns within the conduit. Additionally, it's a cost-effective approach as it utilizes existing infrastructure without the need for purchasing or sourcing new materials.

The other options may not provide the same effectiveness or safety. An extra pulling rope could work but might not always fit or may require additional management. A section of fiber-optic cable might be inappropriate due to its potentially fragile nature, whereas a metal wire could risk damaging either the existing wiring or the conduit itself. Therefore, the chosen answer optimizes both efficiency and safety when working with conduits.