The correct answer is (b), Australia.
A megadiverse country is a nation that contains at least 1,500 species of vascular plants as well as a high number of endemic species. Australia is home to over 300,000 species of plants and animals, of which over 80% are found nowhere else on Earth. The country’s unique biodiversity is due to its long isolation from other landmasses, as well as its diverse climate and geography.
New Zealand (a) is also a megadiverse country, with over 1,000 species of native plants and animals. However, its biodiversity is not as extensive as Australia’s, as the country is much smaller and has a more temperate climate.
Nepal (c) is a biodiversity hotspot, but it is not a megadiverse country. The country is home to over 8,000 species of plants and animals, but it does not have the same level of endemism as Australia or New Zealand.
Austria (d) is not a megadiverse country. The country is home to over 4,000 species of plants and animals, but it does not have the same level of biodiversity as the other countries listed.