Which one is the basic unit of classification?

The correct answer is D. Species.

A species is a group of organisms that can interbreed and produce fertile offspring. It is the basic unit of biological classification.

A division is a taxonomic rank used in some systems of biological classification. It is typically used to group together related classes.

A family is a taxonomic rank used in some systems of biological classification. It is typically used to group together related genera.

A class is a taxonomic rank used in some systems of biological classification. It is typically used to group together related orders.

In conclusion, the basic unit of classification is the species.