The Biosphere Reserve `Dehang Debang` is located in Arunachal Pradesh. A biosphere reserve is an area that aims to promote the conservation of biodiversity and the sustainable use of natural resources. It provides a natural habitat for various species and also supports the livelihoods of local communities.

Option 1, Arunachal Pradesh, is the correct answer because the Dehang Debang Biosphere Reserve is indeed located in this state. Arunachal Pradesh is a northeastern state of India known for its rich biodiversity and scenic beauty.

Option 2, Meghalaya, is incorrect as the Dehang Debang Biosphere Reserve is not located in this state. Meghalaya is another northeastern state known for its beautiful hills and waterfalls.

Option 3, Mizoram, is incorrect as well. Mizoram is also a northeastern state but the Dehang Debang Biosphere Reserve is not located here.

Option 4, Sikkim, is also incorrect. Sikkim is a small state in the Himalayas, but the Dehang Debang Biosphere Reserve is not located here either.

In conclusion, the correct answer is option 1 - Arunachal Pradesh.