The correct answer is option D, Ganga.

Explanation:
The Ganga, also known as the Ganges, is a major river in the Indian subcontinent that flows through the kingdom of Magadha. Here is a detailed explanation of why the Ganga and its tributaries flow through the kingdom of Magadha:

1. Geographical Location:
The kingdom of Magadha was situated in the eastern part of the Indian subcontinent, in present-day Bihar and parts of Uttar Pradesh. The Ganga River flows through this region, making it an important waterway for the kingdom.

2. Importance of the Ganga River:
The Ganga River is one of the most sacred rivers in Hinduism and is considered the lifeline of the region. It has been a significant source of water, transportation, and irrigation for centuries. The fertile plains along the banks of the Ganga have supported agriculture and human settlements, making it an ideal location for the kingdom of Magadha to flourish.

3. Tributaries of the Ganga:
The Ganga River has several tributaries, which are smaller rivers or streams that flow into it. Some of the important tributaries of the Ganga include the Yamuna, Son, Gandak, Kosi, and Ghaghara rivers. These tributaries not only contribute to the flow of water in the Ganga but also bring fertile soil and nutrients to the region, making it suitable for agriculture.

4. Historical Significance:
The kingdom of Magadha was an important political and cultural center in ancient India. It was the birthplace of several influential dynasties, such as the Mauryas and the Guptas. The Ganga River played a crucial role in the development of trade and commerce in the region, as it provided a means of transportation for goods and people. The river also facilitated cultural exchange and the spread of religion, with many important religious sites located along its banks.

In conclusion, the Ganga River and its tributaries flow through the kingdom of Magadha due to its location in the eastern part of the Indian subcontinent. The river has been of great significance to the region, providing water, transportation, and fertile land for agriculture. Its historical and cultural importance has made it an integral part of the kingdom's development and prosperity.