The correct answer is A. 1-2 cm.
The Bihar plain is a vast, low-lying plain in eastern India. It is bounded by the Himalayas to the north, the Vindhya Range to the south, the Ganges River to the east, and the Aravalli Range to the west. The plain is made up of alluvial soil deposited by the Ganges and its tributaries. It is a fertile region and is home to a large population.
The average slope of the Bihar plain is very gentle. It ranges from 1 to 2 cm per kilometer. This means that for every kilometer you travel, the elevation of the land changes by only 1 to 2 cm. This gentle slope makes the Bihar plain an ideal region for agriculture.
The other options are incorrect because they are too steep. A slope of 5 to 6 cm per kilometer is considered to be a moderate slope. A slope of 15 to 16 cm per kilometer is considered to be a steep slope. A slope of 47 to 48 cm per kilometer is considered to be a very steep slope.