The correct answer is option 'D' - Indo-Gangetic Plain.

Explanation:

Introduction to plains
Plains are vast, relatively flat areas of land that are usually located near sea level. They are characterized by fertile soil and are often highly populated due to their favorable conditions for agriculture and human settlement.

Overview of the Indo-Gangetic Plain
The Indo-Gangetic Plain, also known as the Indus-Ganga Plain, is a large plain located in South Asia. It stretches across the northern and eastern parts of India, as well as parts of Pakistan, Nepal, and Bangladesh. The plain is formed by the alluvial deposits of the Indus, Ganges, and Brahmaputra rivers, making it one of the most fertile regions in the world.

Population of the Indo-Gangetic Plain
The Indo-Gangetic Plain is home to a massive population. With an estimated population of over 1.5 billion people, it is the most populated region in the world. This densely populated plain includes major cities such as Delhi, Kolkata, and Lahore. The fertile soil, abundant water resources, and favorable climate support intensive agriculture, leading to high agricultural productivity and attracting a large number of people to settle in this region.

Reasons for high population
Several factors contribute to the high population in the Indo-Gangetic Plain:

1. Fertile soil: The alluvial soil deposited by the rivers in the plain is highly fertile, supporting the cultivation of multiple crops throughout the year. This abundance of agricultural resources attracts farmers and agricultural workers to settle in the region.

2. Water resources: The Indus, Ganges, and Brahmaputra rivers provide a consistent water supply for irrigation, allowing farmers to cultivate crops year-round. This availability of water resources further contributes to the high agricultural productivity and supports a large population.

3. Favorable climate: The climate in the Indo-Gangetic Plain is generally favorable for agriculture, with distinct seasons and moderate temperatures. This enables the cultivation of a wide range of crops and encourages settlement.

4. Economic opportunities: The large population in the Indo-Gangetic Plain creates a demand for various goods and services, leading to the establishment of industries, businesses, and job opportunities. This economic activity attracts people from neighboring regions, further contributing to the population growth.

In conclusion, the Indo-Gangetic Plain is the most populated region in the world due to its fertile soil, abundant water resources, favorable climate, and economic opportunities. These factors have attracted a significant number of people to settle in this region, making it densely populated.