According to IUCN Red List, during the last two decades , the maximum increase in the number of threatened species is among
A) Mammals
B) Amphibians
C) Reptiles
D) Birds

Hint: IUCN stands for International union for conservation of nature. Red list includes all those species which are threatened. According to IUCN more than 27000 species are threatened with extinction and among them the threat to amphibians is most.

Complete answer:
Due to various human activities, the biological wealth of our planet is declining very rapidly.
According to the IUCN Red List, There are about 784 species which include 338 Vertebrates, 359 invertebrates and about 87 plants) which are already extinct because of human greed in the last 500 years. Example: dodo (A large flightless bird) from Mauritius, Quagga (a species of Zebra) from Africa, Thylacine (a marsupials) from Australia, Steller's sea cow from Russia and three subspecies of tiger from Bali, Javan and Caspian.
And there are many more species which are at the verge of extinction (they are more vulnerable for extinction in near future). About 15,500 species are facing the threat of extinction. Among them about 12 percent are birds, 23 percent are mammals, 32 percent are amphibians and about 31 percent are gymnosperms.

Hence, the correct answer is option (B)

Note: There are mainly four cause of extinction (called ‘The Evil Quartet’)
1) Habitat loss and fragmentation: This is the most important cause of extinction. When forests are cut then many animals residing in that forest lose their home. Tropical rainforest once covering more than 14 percent , are now confined to onlr 6 percent. This is due to massive destruction of forest.
2) Over-exploitation: Nature has provided us enough, but when need turns to greed then overexploitation occurs. Over exploitation of natural resources leads to extinction of many species like steller's sea cow, passenger pigeon.