The basic smallest unit of classification is

Explanation-


Taxonomy is the branch of biology that deals with the classification of living organisms into a taxonomical hierarchy. Carolus Linnaeus is known as the Father of Taxonomy-
The taxonomical hierarchy includes-

For Plantae
Kingdom     ( Highest category)                                              
↓
Division/Phylum ( Classes having common characters comes under the same division/phylum)
↓
Class(Orders with similar characters are placed under the same class)
↓
Order(Families having common characters forms the order)
↓
Family(Genera having common characters forms the family)
↓
Genus(Group of closely-related species)
↓
Species (smallest unit of classification)



Final Answer-

So, The correct option is A.
The basic smallest unit of classification is-Species.