- The correct answer is Option 1: Ionic interaction with calcium ions in hydroxyapatite.

- Glass ionomer cement (GIC) is unique among dental restorative materials due to its ability to form a chemical bond with the tooth structure, specifically enamel and dentin.
- This bonding is primarily achieved through an ionic interaction between the carboxyl groups of the polyalkenoic acid in the GIC and the calcium ions present in hydroxyapatite crystals, which are the main mineral components of the tooth.
- This interaction creates a stable and durable bond that enhances the adhesion of the restorative material to the tooth.

Other options are incorrect for the following reasons:
- Hydrogen bonding with collagen fibrils (Option 2) is more relevant to materials that interact primarily with the organic matrix, but GIC bonding is mainly with the mineral component, not collagen.
- Mechanical interlocking through micromechanical retention (Option 3) is a mechanism typical of resin-based composites that rely on etching and resin infiltration, rather than chemical bonding.
- Van der Waals forces (Option 4) are weak physical forces and do not contribute significantly to the adhesive strength of GIC.

Therefore, the key feature enabling GIC to chemically bond to teeth is its ability to form ionic bonds with the calcium ions in hydroxyapatite.

Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition, Volume 1, Chapter 6: Restorative Materials/Page 150