Afatty acid that lacks at least one hydrogen along its chain is considered to be

Venn diagram is missing in the question, however, a venn diagram has been created to illustrate what is being referred to in the question.

Adapted to its environment

Explanation:

The venn diagram being referred to in the question above is missing, however, we can make a very good guess as to which of the labels listed in the question belongs to the region that is said to be marked X.

Rest assured, every species of organism must possess unique characteristics or traits to be well adapted to its environment. So therefore, considering the labels given in the question, the labels that would most likely belong in the region marked X would be “adapted to its environment.

The region marked X should be the region where the 2 circles represented in the venn diagram overlaps. Meaning, both Fish A and Fish B, both share a common attribute which can be labeled as "Adapted to its environment”, as both Fishes dwell in different environment with different conditions. Each type of fish would possess special traits listed in the other labels that would make them adapt well and survive in their different environment.

See the diagram that is attached below, which gives us a mental picture comparing both Fishes.