What change may appear in iron-rich rocks due to oxidation?

Correct answer: Reddish-brown colour

The direct answer is C, reddish-brown colour. Oxidation is a chemical reaction in which a mineral reacts with oxygen. Iron-bearing minerals can form iron oxides, similar to rust on iron objects. Step by step: oxygen and moisture reach iron-containing minerals; the iron reacts and produces oxide minerals; these commonly give the rock a red, brown, or yellowish-brown appearance; the reaction may also weaken the rock. Option C is correct because a rust-like reddish-brown colour is a common visible sign of oxidation in iron-rich rocks. Option A is wrong because blue transparency is not the usual result of iron oxidation. Option B is wrong because oxidation is a chemical alteration, not chemical inactivity. Option D is wrong because iron-bearing rocks do not normally evaporate completely through oxidation. Some rocks may show different shades depending on minerals and moisture, so ‘may appear’ is appropriately cautious. Memory cue: iron plus oxygen often means rust-coloured weathering.