Biodiversity is measured by

a
examining the usefulness of a specific trait in a species that interacts within an ecosystem.
b
counting the total number of one species population and how they interact within an ecosystem.
c
counting the number and variation of different species of plants, animals, and other organisms that interact within an ecosystem.
d
only analyzing animal species that interact within an ecosystem.

c - Biodiversity is measured by counting the number and variation of different species of plants, animals, and other organisms that interact within an ecosystem.