The correct answer is: C. Terai region
The Terai region is the most humid and rainy region of Bihar. It is located in the southern part of the state, along the border with Nepal. The Terai region is a lowland area that is covered in forests and grasslands. The climate of the Terai region is tropical monsoonal, with a long, hot, and humid summer and a short, cool, and dry winter. The average annual rainfall in the Terai region is about 1,500 mm.
The Ganga-Diara region is located in the northern part of Bihar. It is a floodplain region that is formed by the confluence of the Ganges and the Yamuna rivers. The Ganga-Diara region is a relatively dry region, with an average annual rainfall of about 800 mm.
The Gandak-Kosi Doab is located in the central part of Bihar. It is a region that is located between the Gandak and Kosi rivers. The Gandak-Kosi Doab is a relatively dry region, with an average annual rainfall of about 1,000 mm.
The Ghaghara-Gandak region is located in the eastern part of Bihar. It is a region that is located between the Ghaghara and Gandak rivers. The Ghaghara-Gandak region is a relatively dry region, with an average annual rainfall of about 800 mm.