Option d is the correct answer An IIT Kharagpur study claims that a drought-like phase that lasted for over 900 years ended the Indus Valley Civilisation. The study showed that around 4,350 years ago, monsoon weakened under the influence of strong El Niño and southward migration of the inter-tropical convergence zone. This decreased moisture transport and snow deposition in northwest Himalaya, which was a major source of water of the Indus and its tributaries. Important Tips Different Theories about end of Indus valley Civilization: According to British archaeologist Mortimer Wheeler, a nomadic Indo-European tribe called the Aryans invaded and conquered the Indus River Valley. Some experts believe the drying of the Saraswati River, which began around 1900 BCE, was the main cause for climate change, Some scholars believe that a great flood struck the area and caused the end of civilization.