The question pertains to the archaeological understanding of the mature phase of the Harappan Civilization, particularly the timeline of its sites in the Cholistan region. Cholistan is currently in Pakistan and is known for several Harappan sites. Understanding when these sites were abandoned is crucial for comprehending the decline of the Harappan Civilization.

To determine the correct answer, we must focus on the historical timeline of the Harappan Civilization and facts known from archaeological research:

1. The Mature Harappan Phase is considered to be from approximately 2600 BCE to 1900 BCE.
2. The decline of the Harappan civilization is generally marked from about 1900 BCE onward, with significant changes in settlement patterns and material culture.
3. During this period, many Harappan sites, including those in Cholistan, began to be abandoned.

Based on these archaeological insights, the correct option is:

• Option: C. 1800 BCE

This date corresponds well with the decline phase of the Harappan Civilization. It indicates that by around 1800 BCE, many of the mature Harappan sites in the Cholistan region had been abandoned, reflecting this broader transitional period in ancient Indian history.

Thus, considering the archaeological findings and the timeline of the Harappan Civilization, the sites' abandonment around 1800 BCE is well-supported by historical evidence. This conclusion also aligns with the fact that many other Harappan sites across the broader region were experiencing similar declines during this period.