A) A rare species is a species that is very uncommon, scarce, or infrequently encountered.
B) A species which has died out and is no more found in the wild is known as extinct 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 species which is very likely to become extinct is known as endangered species.
So, the correct answer is 'Vulnerable'.