Option 1 matches Aeration followed by filtration (Process A) with Removal of colour (Contaminant 1). This process involves introducing air into the water to help remove any dissolved gases or volatile compounds that may cause coloration. The water is then filtered to remove any particles or sediments that may contribute to coloration.

Option 2 matches Reverse Osmosis (Process B) with Removal of excess salts (Contaminant 4). Reverse osmosis is a process in which water is forced through a semi-permeable membrane to remove dissolved salts and other impurities.

Option 3 matches Filtration through activated carbon (Process C) with Removal of iron (Contaminant 3). Activated carbon is known for its ability to remove iron from water by adsorbing the iron particles onto its surface.

Option 4 matches Treatment with UV light (Process D) with Removal of microorganisms (Contaminant 2). UV light treatment is an effective method for disinfecting water by inactivating or killing microorganisms, such as bacteria and viruses.

The correct answer is option 3, which matches the processes with the contaminants correctly.