The basic unit of classification is the species. Let's break down the answer into heading and HTML bullet points for a more detailed explanation:

- Classification is the process of organizing and categorizing living organisms into different groups based on their characteristics and relationships.
- It helps scientists to study and understand the vast diversity of life on Earth.

- The Linnaean classification system, developed by Carl Linnaeus in the 18th century, is the most widely used system for classifying living organisms.
- It uses a hierarchical structure, with each level representing a different category of classification.

- The Linnaean system includes several levels of classification, from broad to specific categories.
- The levels, in descending order, are: Kingdom, Phylum, Class, Order, Family, Genus, and Species.
- Each level represents a progressively smaller and more specific group of organisms.

- The species is the most specific and fundamental unit of classification.
- It refers to a group of organisms that share similar characteristics and can interbreed to produce fertile offspring.
- Members of the same species have a high degree of genetic similarity.

- While genus is a level in the Linnaean system, it is not the basic unit of classification.
- The genus is a broader category that includes closely related species.
- Order is also a level in the Linnaean system, but it is even higher and more inclusive than the genus level.

- The basic unit of classification is the species.
- The Linnaean classification system includes various levels, but species is the most specific and fundamental category.
- The species level represents a group of organisms with similar characteristics and the ability to interbreed.

Basic unit or smallest unit of taxonomy/ classification is species. Species are a group of organisms that are morphologically similar and can interbreed. So, the correct answer is 'species'