Civil war between Harappan families is not a widely held belief for the decline of the Harappan civilization; theories mainly focus on environmental changes, external invasions, and regional disruptions. therefore, option C is correct.
The widely held beliefs regarding the collapse of the Harappan civilization include disrupted farming and trading patterns, environmental disaster, and invasion by Aryans to the north. However, the option that is NOT widely held as a reason for the Harappan civilization's fall is C. Civil war between Harappan families.
Scholars have discussed various theories, such as climate change leading to environmental changes, tectonic shifts altering river courses, disease, and environmental degradation due to deforestation and soil salinity. The Aryans' invasion is also mentioned in historical texts, although its direct impact on the Harappan decline is debated among historians.