The correct answer is A. East Champaran.
Sal mixed moist deciduous forests are a type of tropical moist deciduous forest found in the Indian subcontinent. They are characterized by a mixture of sal (Shorea robusta) trees, along with other deciduous trees such as teak (Tectona grandis), bamboo, and a variety of shrubs and herbs. These forests are found in a belt that runs from the foothills of the Himalayas in the north to the Deccan Plateau in the south.
The East Champaran district of Bihar is one of the most important areas for sal mixed moist deciduous forests in India. The forests in this district are home to a variety of wildlife, including tigers, leopards, elephants, and deer. The forests are also an important source of timber and other forest products.
The other options are incorrect because they are not major areas for sal mixed moist deciduous forests. Bhagalpur and Gaya are districts in Bihar, but they do not have as much sal mixed moist deciduous forest as East Champaran. West Champaran is a district in Bihar that is located west of East Champaran. It does have some sal mixed moist deciduous forest, but it is not as extensive as the forests in East Champaran.