Assertion: By 2600 BCE most of the Mature Harappan sites in regions such as Cholistan had been abandoned.
Reasoning: Simultaneously, there was an expansion of the population into new settlements in Gujarat, Haryana and western Uttar Pradesh.

The Assertion is incorrect but the Reason is correct.

The correct answer is Option 4 - The Assertion is incorrect but the Reason is correct.

The asertion- By 2600 BCE most of the Mature Harappan sites in regions such as Cholistan had been abandoned is incorrect as there is evidence that by c. 1800 BCE most of the Mature Harappan sites in regions such as Cholistan had been abandoned.

There is evidence that by c. 1800 BCE most of the Mature Harappan sites in regions such as Cholistan had been abandoned (Correct version of Assertion). Simultaneously, there was an expansion of population into new settlements in Gujarat, Haryana and western Uttar Pradesh (Reason). In the few Harappan sites that continued to be occupied after 1900 BCE there appears to have been a transformation of material culture, marked by the disappearance of the distinctive artefacts of the civilisation – weights, seals, special beads. Writing, long-distance trade, and craft specialisation also disappeared. In general, far fewer materials were used to make far fewer things. House construction techniques deteriorated and large public structures were no longer produced. Overall, artefacts and settlements indicate a rural way of life in what are called “Late Harappan” or “successor cultures”.