These are longitudinal valleys formed as a result of

Solution

Option b is correct. Duars are alluvial floodplains found in Darjeeling and Sikkim HImalayas. They are the result of folding during the formation of the Himalayas. The moderate slope, thick soil cover with high organic content, uniform year-round rainfall with mild winters paved way for their development into tea plantations by the British. They are known for their scenic beauty and are dotted with tourist destinations such as Darjeeling and Siliguri. They are home to many endangered species like one horned rhino and tiger. National parks and wildlife sanctuaries are found in large numbers such as Manas, Jaldpara, Buxa National Parks and Mahananda Wildlife Sanctuary.