Every unit of classification regardless of its rank is
Taxon
The correct option is : A
Explanation of correct option:
Explanation of incorrect options:
Option B:
In classification, variety is a rank of particular species.
Option C:
In hierarchical classification, a species is a basic unit for classification and also it is the lowest rank.
Option D:
Strain is also a taxonomic rank which is present on the downside of species as it is a low level of classification.
Final Answer: Every unit of classification regardless of its rank is Taxon.