Which of the following does not belong to the list of biosphere reserves that have been set up in India so far?

Introduction / Context:
This question is about biosphere reserves in India. Biosphere reserves are large areas of biodiversity conservation that integrate protection with sustainable use by local communities. They are often part of UNESCO's Man and the Biosphere Programme. Exams frequently ask which sites are biosphere reserves and which names are incorrect or fictitious.

Given Data / Assumptions:
- The options list Nilgiris, Sundarbans, Nanda Devi, Reuna and Gulf of Mannar.
- We are asked to identify the option that is not a biosphere reserve in India.
- We assume the standard list of Indian biosphere reserves notified by the Government of India.
- Spelling variations are considered but the core names should match known reserves.

Concept / Approach:
India has several biosphere reserves such as Nilgiri, Nanda Devi, Sundarbans, Gulf of Mannar, Pachmarhi and others. Names like Nilgiris (commonly referred to as Nilgiri Biosphere Reserve), Sundarbans, Nanda Devi and Gulf of Mannar are all established biosphere reserves. "Reuna" does not correspond to any recognised biosphere reserve; it appears to be a fictitious or incorrect name. The approach is to match each option with the known list and identify the one that does not fit.

Step-by-Step Solution:
Step 1: Recall that Nilgiris was India's first biosphere reserve, covering parts of Tamil Nadu, Kerala and Karnataka.Step 2: Remember that Sundarbans, spanning parts of India and Bangladesh, is a famous mangrove ecosystem and is designated as a biosphere reserve.Step 3: Nanda Devi Biosphere Reserve focuses on the high Himalayan region around Nanda Devi National Park in Uttarakhand.Step 4: Gulf of Mannar is a marine biosphere reserve on the southeast coast of India, between India and Sri Lanka.Step 5: Check the remaining option, Reuna, which does not correspond to any officially declared biosphere reserve in India.Step 6: Therefore, Reuna is the name that does not belong to the list of Indian biosphere reserves.

Verification / Alternative check:
You can verify this by consulting a standard list of Indian biosphere reserves in geography textbooks or government publications, which will include Nilgiri, Nanda Devi, Sundarbans and Gulf of Mannar among others. No such list includes any area called Reuna. Exam guides that summarise biosphere reserves also confirm that the other four options are genuine reserves, while Reuna is not.

Why Other Options Are Wrong:
Nilgiris: A genuine biosphere reserve in the Western Ghats, shared by three southern states.

Sundarbans: A real mangrove biosphere reserve in the delta of the Ganga, Brahmaputra and Meghna rivers.
Nanda Devi: A real biosphere reserve centred on Nanda Devi National Park in Uttarakhand.
Gulf of Mannar: A real marine biosphere reserve along the Tamil Nadu coast.

Common Pitfalls:
Candidates sometimes doubt spellings or think that names like Gulf of Mannar refer only to marine areas and not to biosphere reserves, which may lead them to choose wrongly. Another pitfall is assuming that any unfamiliar name, like Reuna, might represent a lesser known reserve. The best prevention is to learn the official list of Indian biosphere reserves well enough to recognise genuine names and quickly filter out invented ones.

Final Answer:
Reuna