The correct option in this case is B: Both A and R are true, but R is not a correct explanation of A.

The assertion states that the Ganga Plain is the most densely-populated part of India, which is true. The Ganga Plain, also known as the Gangetic Plain, is a fertile region spread across the states of Uttar Pradesh, Bihar, West Bengal, and parts of other neighboring states. It is characterized by its rich alluvial soil and extensive irrigation facilities, making it suitable for agriculture. Due to its fertile land and favorable agricultural conditions, the Ganga Plain has attracted a significant population, leading to high population density in the region.

The reason provided is that the Ganga is the most harnessed river of India, which is not a correct explanation for the high population density in the Ganga Plain. While it is true that the Ganga River is one of the most important rivers in India and has been harnessed for various purposes such as irrigation, hydroelectric power generation, and transportation, the reason alone does not directly explain the high population density in the Ganga Plain. Population density is influenced by a variety of factors, including geographical, historical, economic, and cultural factors. The availability of fertile land for agriculture, historical settlement patterns, economic opportunities, and cultural factors play significant roles in determining population density.

Therefore, both the assertion and the reason are true, but the reason provided is not a correct explanation for the high population density in the Ganga Plain.