Concept:Biological classification is a hierarchical system, and species is the most specific, lowest rank within it.
Explanation:Step 1: Remember the standard taxonomic hierarchy used in biology.
Step 2: This is normally written in descending order as: Kingdom → Phylum → Class → Order → Family → Genus → Species.
Step 3: Kingdom is the highest and broadest rank, while lower ranks separate organisms according to shared features and evolutionary relationships.
Step 4: Moving down the hierarchy narrows the group further; the most similar individuals eventually fall into one species.
Step 5: Class appears high above genus, so it is a larger, less specific rank than species.
Step 6: Genus is the rank directly above species, meaning it is also not the lowest level.
Step 7: Since species is the final and narrowest group in the hierarchy, it is the lowest unit of classification.
Answer:D. Species