A) A rare species is a group of organisms that are very uncommon, scarce, or infrequently encountered.
B) A vulnerable species is one which has been categorized by the International Union for Conservation of Nature as likely to become endangered unless the circumstances that are threatening its survival and reproduction improve.
C) In intermediate species is a species that has similarities between an ancestral form and that of its descendants.
D) An indeterminate species is a species which is endangered but with an unknown cause.
So, the correct answer is 'Indeterminate'.