The soldiers of Napolean's army, while on the Alps during freezing winter, suffered a serious problem as regards to the tin buttons of their uniforms. White metallic tin buttons got converted to grey powder. This transformation is related to:

A change in the crystalline structure of tin

The correct answer is (2), A change in the crystalline structure of tin.

Tin has two crystalline allotropes, white tin and grey tin. White tin is the stable form of tin at room temperature, but it can convert to grey tin at low temperatures. Grey tin is a much less dense form of tin, and it is brittle and easily powdered.

The soldiers of Napoleon's army were in the Alps during the freezing winter, so the temperature was below the transition temperature for tin. This caused the white tin buttons on their uniforms to convert to grey tin, which then powdered and fell off.

The other options are incorrect.

• Option (1) is incorrect because nitrogen does not react with tin at low temperatures.
• Option (3) is incorrect because water vapor does not react with tin at low temperatures.
• Option (4) is incorrect because the partial pressure of oxygen in the air does not change at low temperatures.