Hydrological changes were the most suitable reason that made the Harappans move away from their urban settlements.

Explanation:

1. Introduction:
The Harappan civilization, also known as the Indus Valley civilization, was one of the earliest urban civilizations in the Indian subcontinent. It flourished from around 2600 BCE to 1900 BCE in the fertile plains of the Indus River and its tributaries. However, over time, the Harappans gradually abandoned their urban settlements and dispersed to other regions.

2. Hydrological Changes:
Hydrological changes, particularly the shifting course of rivers and changes in the availability of water, played a significant role in the Harappans' decision to move away from their urban settlements. This can be attributed to the following reasons:

a) Shifting River Courses:
The Indus River and its tributaries, which were the lifelines of the Harappan civilization, experienced frequent shifts in their courses. These changes disrupted the agricultural practices and water management systems of the Harappans. The sudden diversion of rivers would have led to water scarcity, affecting their agricultural productivity and overall sustainability.

b) Floods and Droughts:
The shifting river courses also made the settlements vulnerable to frequent flooding and droughts. The unpredictable nature of these natural disasters would have made it challenging for the Harappans to maintain their urban settlements. Floods could destroy crops, damage infrastructure, and displace people, while droughts could lead to food shortages and economic instability.

c) Declining Water Supply:
As the rivers shifted, the availability of water for irrigation and daily use would have decreased in certain regions. The Harappans heavily relied on sophisticated systems of canals and reservoirs to manage their water supply. However, with hydrological changes, these systems would have become ineffective, leading to water scarcity and further impacting their agricultural practices and livelihoods.

3. Conclusion:
In conclusion, hydrological changes, such as shifting river courses, floods, droughts, and declining water supply, were the most suitable reasons that made the Harappans move away from their urban settlements. These changes disrupted their agriculture, water management systems, and overall sustainability, forcing them to disperse to other regions in search of better living conditions.