The Foundation of Any Classification System Is the Species Level

The foundation of any classification system is the species level. Which of the following is accepted as a way of classifying species?

A) Species are based on structural and morphological features of the plant body.
B) Species is a population capable of interbreeding within the population, and reproductively isolated from other groups.
C) Species are based on phylogenetic history; individuals with a common evolutionary background are species.
D) Species are groups of related individuals that occupy unique ecological niches.
E) All of these are currently in use by some segment of the taxonomic community.