a. Insects
b. Mammals
c. Amphibians
d. Reptiles
Answer: c. Amphibians
Explanation: According to the IUCN regional red listing guidelines, 12% of all the birds species, 23% of all mammals species, 31% of all gymnosperms species and 32% of all amphibian species in the world face the threat of extinctions and have a highest percentage of endangered species, and the threats including habitat loss, fragmentation and degradation, pollution, climate change, and invasive alien species. Amphibians are more vulnerable to extinction.