The correct answer is water pollution.
The leading cause of limited water supply is water pollution.The overall amount of water on earth is same. The shortage of water lies when it comes to fresh water is used for domestic purposes.
The main cause of limited supply of water is water pollution. Due to pollution the water left for use is restricted as polluted water cannot be used as potable water or commercial purpose.
The pollutant from industries, house, agricultural, pesticides and oil spills are discharged in water bodies which makes it polluted and unfit for use.