IUCN stands for International Union for Conservation of Nature and Natural Resources. It is the authority on threatened species, and it treats threatened species as a group of three categories, depending on the degree relative to which they are threatened. Those categories are Vulnerable species; Endangered species and Critically endangered species. As per the Red list of 2000, there are 11,096 threatened species.
So, the correct answer is 'International Union for Conservation of Nature and Natural Resources'.