The correct answer is option 3: C. 1800 BCE. This means that most of the mature Harappan sites in the region of Cholistan were abandoned around 1800 BCE.

The Harappan civilization was an ancient civilization that thrived in the Indus Valley region, which includes Cholistan, from around 2600 BCE to 1900 BCE. During this time, the region was known for its well-planned cities, advanced drainage systems, and sophisticated culture.

Based on archaeological evidence, it is believed that around 1800 BCE, the mature Harappan sites in Cholistan started to decline and eventually became abandoned. The reasons for this decline are still debated among scholars but could include environmental changes, such as changes in climate patterns or changes in the course of the rivers that provided water to the cities.

It is important to note that while most of the mature Harappan sites in Cholistan were abandoned around 1800 BCE, there were still some smaller settlements and evidence of cultural continuity in the region after that time. However, the major urban centers of the Harappan civilization had already declined by this period.

In summary, option 3, C. 1800 BCE, is the correct answer as it represents