Define Biosphere reserves?
A Biosphere Reserve is a unique and representative ecosystem of terrestrial and coastal areas which are internationally recognised within the framework of UNESCO's Man and Biosphere (MAB) Programme.
These reserves aim at achieving three objectives related to conservation. For the purpose of effective conservation of flora and fauna, special steps have been initiated by the Government of India in collaboration with UNESCO's Man and Biosphere Programme.
There are 18 Biosphere Reserves in India, out of which twelve have been recognised by UNESCO on the World Network of Biosphere Reserves.
Explanation
The answer is directly derived from the textbook definition which states that Biosphere Reserves are unique ecosystems of terrestrial and coastal areas recognised under UNESCO's MAB Programme. The context also mentions that there are 18 Biosphere Reserves in India with 12 recognised by UNESCO, and that these are meant for effective conservation of flora and fauna.