According to the IUCN Red List, during the last two decades, the maximum increase in the number of threatened species is among amphibians as some groups like amphibians appear to be more vulnerable to extinction. More than 15,500 species worldwide are facing the threat of extinction. Presently, 12 percent of all bird species, 23 percent of all mammal species, 32 percent of all amphibian species and 31per cent of all gymnosperm species in the world face the threat of extinction.
So, the correct answer is 'Amphibians'.