In layout (marking-out) work the metal surface is first coated with a marking medium so that lines scribed on it stand out clearly against the bare metal.

The medium must dry to a thin, even film of contrasting colour that the scriber can cut through, revealing the bright metal line beneath.

Red lead is used as such a marking medium. Brushed thinly on the surface, its strong red colour provides a background against which the fine scribed lines show up sharply during marking out.

Its whole purpose in this context is the visibility of the layout lines, so the worker can see and follow the marked outline accurately.

It is not applied as a lubricant for measuring instruments (option 1); a marking coat would in fact hinder, not ease, instrument movement.

It is not a cleaning agent (option 2) nor a rust-protective storage coating for tools (option 4); those are unrelated tasks handled by other materials.

Red lead's role is purely to make scribed lines visible on the layout surface.

Hence, the answer is Red lead is applied to surfaces to make scribed lines more visible during layout work.