Which of the following statements about species, as defined by the biological species concept, is (are) correct? i. biological species are defined by reproductive isolation. ii. biological species are the model used for grouping extinct forms of life. iii. the biological species is the largest unit of population in which successful reproduction is possible.

The correct answer is 1) biological species are defined by reproductive isolation and 3) biological species are the largest unit of population in which successful reproduction is possible.

According to biological species concept, two organisms belong to same species if they are capable of interbreeding and producing viable fertile offsprings.

This means that group of these members belonging to the same species is reproductively isolated from other group of species. Thus, according to this concept, successful reproduction is possible.

Therefore, option 1) and 3) is the right answer.