Almost all terrestrial species spend their entire lives within a narrow zone that extends from a few tens of meters above the tops of the trees to a few meters below Earth's surface. Therefore, terrestrial organisms are relatively easy to study, as they are readily accessible and visible to the scientist. In contrast, ocean life is present throughout the depth of the ocean waters, and for several meters, or more, into the sediment.(Douglas A Segar, Introduction to Ocean Sciences)

A. Argument; conclusion: They are readily accessible ... scientist.
B. Argument; conclusion: Almost all terrestrial species ... surface.
C. Argument; conclusion: In contrast, ocean life ... the sediment.
D. Argument; conclusion: Terrestrial organisms are relatively easy to study.
E. Nonargument.

Hello, you didn't say what the task is, but I assume we have to choose the option that best describes the composition and the format of the excerpt from Douglas A Segar's Introduction to the Ocean Sciences. If so, I would choose D.

Explanation:

I chose D because the excerpt begins with the argument and continues with the conclusion, that is that "terrestrial organisms are relatively easy to study". Then, the study of terrestrial organisms are compared with the study of ocean life, that is not so easy due to its nature of being present "throughout the depth of the ocean waters, and for several meters, or more, into the sediment." and it is not as easy to reach as the first one.