During chemical weathering, when minerals in feldspar change to kaolinite clays, the process is termed as :

When feldspar minerals (like orthoclase) react with water, the H+ ions from water replace cations such as K+, producing kaolinite clay along with dissolved potassium and silica. This addition/absorption of water into the mineral structure is called hydration (or more specifically hydrolysis, which is grouped under hydration processes in standard physical geography texts used by CDS/UPSC). Carbonation involves CO2-rich water dissolving carbonate rocks; oxidation involves O2 reacting with metals like iron; chelation involves organic acids. Hence the correct process is hydration.