The correct answer is option 'C' which states that Mehrgarh was not a Harappan city.

Explanation:

1. The Harappan Civilization, also known as the Indus Valley Civilization, was one of the earliest urban civilizations in the world. It developed around 2600 BCE in the northwest region of the Indian subcontinent.

2. The Harappan Civilization is known for its well-planned cities, advanced architecture, and sophisticated drainage systems. The cities were characterized by a grid-like layout with well-constructed brick buildings.

3. There were several prominent cities of the Harappan Civilization, and among them, Mehrgarh was not one of them. Mehrgarh is an archaeological site located in present-day Pakistan, near the Bolan Pass.

4. Mehrgarh is significant in its own right as it predates the Harappan Civilization. The site has evidence of early agricultural practices and animal domestication, indicating the transition from a hunter-gatherer lifestyle to settled farming communities.

5. The Harappan cities, on the other hand, flourished during the Mature Harappan phase (2600-1900 BCE). Some of the well-known Harappan cities include:

- Lothal: Located in present-day Gujarat, India, Lothal was an important port city and a center of trade and commerce. It had a sophisticated dockyard and is known for its expertise in maritime activities.

- Dholavira: Situated in present-day Gujarat, India, Dholavira is one of the largest Harappan cities. It is known for its well-preserved city planning, including a large water reservoir and an intricate system of water channels.

- Sokhta Koh: Sokhta Koh, also known as Shahr-e Sukhteh, is an archaeological site located in present-day Iran. It was a significant trading center during the Bronze Age, but it is not considered a Harappan city.

6. Therefore, out of the given options, Mehrgarh is the correct answer as it predates the Harappan Civilization, while the other options (Lothal, Dholavira, and Sokhta Koh) were indeed Harappan cities.

In conclusion, Mehrgarh was not a Harappan city, and it is important to note its significance in the pre-Harappan era.