Reasons for the Harappans moving away from their urban settlement:

There are several theories and explanations for why the Harappans, who were part of the Indus Valley Civilization, abandoned their urban settlements. Among these, the most suitable reason is hydrological changes.

1. Hydrological changes:
- The most significant reason for the decline and eventual abandonment of Harappan cities is believed to be hydrological changes, particularly the drying up of the Ghaggar-Hakra River system.
- The Indus Valley Civilization was heavily reliant on an elaborate system of canals and irrigation to support their agricultural practices. However, over time, the Ghaggar-Hakra River, which supplied water to these canals, experienced a decline in its flow.
- This hydrological change led to a decrease in the availability of water for irrigation, resulting in a decline in agricultural productivity and food scarcity. This would have put immense pressure on the urban settlements and forced the Harappans to seek alternative locations with better access to water resources.

2. Ecological changes:
- Ecological changes, such as deforestation and soil degradation, may also have played a role in the decline of the Harappan civilization.
- The extensive use of timber for construction and fuel, coupled with improper land management practices, could have led to deforestation and soil erosion, reducing the fertility of agricultural lands.
- This ecological degradation would have further exacerbated the food crisis faced by the Harappans and contributed to their decision to move away from their urban settlements.

3. Foreign invasion:
- While there is evidence of violence and destruction in some Harappan sites, the theory of foreign invasion as the primary cause of their decline is not supported by substantial evidence.
- The absence of extensive fortifications and the lack of clear evidence of large-scale warfare suggest that foreign invasion alone cannot explain the abandonment of the urban settlements.

4. Demographic changes:
- Demographic changes, such as population growth and migration, may have also played a role in the decline of the Harappan civilization.
- As the population increased, the existing resources and infrastructure might not have been able to support the growing number of people.
- This could have led to increased competition for resources and social unrest, prompting some groups to migrate to new areas in search of better living conditions.

Conclusion:
While there are various factors that contributed to the decline of the Harappan civilization, including ecological changes, foreign invasion, and demographic changes, the most suitable reason is the hydrological changes, particularly the drying up of the Ghaggar-Hakra River system. The decrease in water availability for irrigation would have severely impacted agriculture and food production, leading to a decline in the sustainability of the urban settlements and ultimately forcing the Harappans to move away in search of better water resources.