Final answer:
The morphological-species concept classifies organisms on the basis of their physical characteristics (Option A), specifically their morphology, which includes the observable traits known as the phenotype.
Step-by-step explanation:
The morphological-species concept classifies organisms based on their physical characteristics.
Morphology, in biology, refers to the form and structure of organisms and is used for classification purposes, highlighting observable physical traits.
Organisms are determined to belong to the same species if they appear similar to one another, suggesting that they share a common lineage or ancestry.
This approach is distinct from other species concepts, which may utilize genetic makeup, geographic distribution, or behavioral traits as classification criteria.
Organisms in the animal kingdom are often classified based on their body morphology and their developmental pathways.
A species comprises a group of individual organisms with significant genetic similarities and shared external and internal characteristics, which are often evident through their morphological features.
The observable traits expressed by an organism, such as hair color, skin color, height, and build, are described as its phenotype, whereas the genotype refers to the genetic makeup inherited from the organism's parents.
Hence, the correct answer is: physical characteristics (Option A).