"They"at the starting part of the sentence refers to the krill. skrill is singular, we need "it" This eliminates A and C
The elimination on the rest depends on the meaning of the sentence.
In short sentence in B reads as follows:
The krill represents one of many organism survival story and it has set itself up as a highly adaptive creature in an age when to be able to adapt to the climatic alteration.
The above sentence does not make sense as the ability is trying to modify/represent krill. However, the intent is to tell the general ability of all creatures
similarly sentence D reads as:
The krill represents one of many organism survival story and it has set itself up as a highly adaptive creature in an age when its ability to adapt to the climatic alteration.
same error as in B
Sentence E:
The krill represents one of many organism survival story and it has set itself up as a highly adaptive creature in an age when the ability to adapt to the climatic alteration.
here the ability is representing a general situation
The krill, an abundant shrimp-like marine crustacean that inhabits the bottom of oceans, represents one of many organism survival story: along with the huge sized whales, the star fish, and other species that cohabit the ocean,
they have set themselves up as highly adaptive creatures in an age when the capability to adapt to the climatic alteration introduced by Earth's ever changing landscape has created a successive group of dominant sea creatures.
(A) they have set themselves up as highly adaptive creatures in an age when the capability
(B) it has set itself up as a highly adaptive creature in an age when to be able
(C) they have set themselves up as being highly adaptive creatures in an age when being able
(D) it has set itself up as a highly adaptive creature in an age when its ability
(E) it has set itself up as a highly adaptive creature in an age when the ability