The correct option is Chemical reaction of the oxide on rock surface.
Explanation
Prehistoric rock paintings, particularly those from the Mesolithic period found in sites like Bhimbetka, utilize natural mineral pigments. The remarkable survival of these artworks over thousands of years is attributed to specific chemical properties and interactions.
Detailed Analysis:
- Chemical reaction of the oxide on rock surface is Correct: The artists primarily used mineral pigments such as haematite (iron oxide) for red and limestone or kaolin for white. The longevity of these pigments is primarily attributed to the chemical reaction of the oxide present in the colour with the rock surface. Over time, the mineral oxides form a stable chemical bond with the rock substrate, effectively becoming an integral part of the stone, which resists weathering and fading.
- Repeated repainting by later groups is Incorrect: While superimposition (painting new layers over old ones) is a common feature in rock art shelters, it explains the sequence of layers rather than the chemical durability of the pigments themselves.
- Extremely dry climate alone is Incorrect: Although the location of paintings in deep niches protects them from direct rain and sunlight, climate alone does not account for the pigment's ability to adhere to the rock for millennia without a modern binder.
- Modern restoration work is Incorrect: The paintings have survived since the prehistoric era naturally; their longevity predates any modern restoration efforts.
Key Takeaway: The durability of prehistoric rock paintings is due to the use of mineral-based pigments (iron and manganese oxides) and their subsequent chemical bonding with the rock surface.