Final answer:
Composite resin is the least resistant to scratching during extrinsic stain removal procedures like selective polishing, compared to amalgam, glass ionomer, and gold foil.
Step-by-step explanation:
The restorative material that is the LEAST resistant to scratching during extrinsic stain removal procedures, like selective polishing, is composite resin. Composite resins have a softer surface in comparison to other restorative materials such as amalgam, glass ionomer, and gold foil, making them more susceptible to scratches during polishing. Amalgam is much harder, while glass ionomer is generally more resistant to scratching but less so than amalgam and gold foil, which is also very resistant due to its inherent metallic properties. Therefore, great care must be taken when polishing composite resin restorations to avoid damaging them.