The correct answer is option 'c) Both 1 and 2'.

Explanation:
The statement consists of two parts. Let's analyze each part separately to understand why both statements are correct.

1. The Ganga-Brahmaputra basin is formed by the tributaries of rivers Ganga and Brahmaputra:
The Ganga-Brahmaputra basin is one of the largest river basins in the world, covering an area of about 1.7 million square kilometers. It is formed by the tributaries of two major rivers - the Ganga and the Brahmaputra.

The Ganga River, also known as the Ganges, is one of the most sacred rivers in India and flows through northern India and Bangladesh. It has several major tributaries such as the Yamuna, Son, Gandak, and Kosi. These tributaries contribute to the flow and formation of the Ganga-Brahmaputra basin.

The Brahmaputra River, on the other hand, is a transboundary river that flows through China, India, and Bangladesh. It is the largest river in Northeast India and has several tributaries such as the Subansiri, Kameng, Manas, and Teesta. These tributaries also contribute to the formation of the Ganga-Brahmaputra basin.

Therefore, it is correct to say that the Ganga-Brahmaputra basin is formed by the tributaries of rivers Ganga and Brahmaputra.

2. The summers are hot and winters are cool in this basin:
The climate of the Ganga-Brahmaputra basin is influenced by its geographical location, topography, and the monsoon winds. The basin experiences a subtropical monsoon climate, characterized by hot summers and cool winters.

During the summer season, which extends from April to June, the temperatures in the basin can reach high levels, often exceeding 40 degrees Celsius in some areas. The hot and dry winds blowing from the northwest, known as the loo, contribute to the high temperatures during this period.

In contrast, the winter season, which extends from December to February, brings cooler temperatures to the basin. The average temperatures during winters range from 10 to 15 degrees Celsius, making the winters relatively cooler compared to the scorching summers.

Therefore, it is correct to say that the summers are hot and winters are cool in the Ganga-Brahmaputra basin.

In conclusion, both statements in the given question are correct. The Ganga-Brahmaputra basin is indeed formed by the tributaries of rivers Ganga and Brahmaputra, and the basin experiences hot summers and cool winters.