Why is species diversity important in an ecosystem?

you must have diversity to have an ecosystem, If you only have one organism that is considered a population.

Explanation:

If you think about the food chain, one species gives it's energy to another. So by having a diversity in species, it allows for an ecosystem to form.

Option D, It is the remains of an organism that lived in a narrow geographic area

Explanation:

Animals or plants that live in a specific time span of earth and are preserved beneath the earth as fossils are known as index fossils. Generally, the time span of their burial is quite small or narrow. Such fossils has following characteristics

a)Distinctive

b)Easy recognizable

c)Abundant

d)Wide range of geographic distribution

e)Short range of time distribution.  

Because of their distinctiveness, these fossils form the basis of defining boundaries in the geologic time scale and correlation of strata.

Hence, option D is correct