A) A species which is vulnerable to endangerment in the near future is known as Threatened species.
B) A species which is very likely to become extinct is known as endangered species.
C) A vulnerable species is a species which is likely to become endangered unless the circumstances which are threatening its survival and reproduction improve.
D) A rare species is a species that is very uncommon, scarce, or infrequently encountered.
So, the correct answer is 'Endangered species'.