Which of the following is unreactive in air on heating ?

Crystalline boron is unreactive in air on heating because it forms a stable oxide layer on its surface when exposed to air. This oxide layer prevents further oxidation of the boron crystal and protects it from further reaction with air.

The oxide layer that forms on the surface of boron is a thin layer of boron oxide (B₂O₃). Boron oxide is a hard and brittle material that is stable at high temperatures and is resistant to further oxidation. This oxide layer acts as a protective coating and prevents any further reaction of the boron crystal with oxygen in the air.

Furthermore, the crystalline structure of boron is very stable and does not easily undergo chemical reactions with other elements or compounds. This is because the boron atoms in the crystal are strongly bonded together in a covalent network, which makes it difficult for other elements or compounds to break the bonds and react with the boron.