Final answer:
Biological species are defined by reproductive isolation and are the largest unit of population in which successful reproduction is possible.
Step-by-step explanation:
The correct statements about species, as defined by the biological species concept, are:
Biological species are defined by reproductive isolation. This means that individuals of different species cannot reproduce successfully with each other, leading to the formation of distinct species.
The biological species is the largest unit of population in which successful reproduction is possible. This means that individuals within the same biological species have the ability to interbreed and produce viable, fertile offspring.
Therefore, the correct answer is option C) I and III.