Threatened species are any species (including animals, plants, fungi, etc.) which are less in number and can be endangered in the near future.
The International Union for Conservation of Nature (IUCN) is the foremost authority on threatened species, and treats threatened species not as a single category, but as a group of three categories, depending on the degree to which they are threatened:
- Vulnerable species
- Endangered species
- Critically endangered species
Thus, the correct answer is option D.