Final answer:
The closest relation between two different species of reptile is at the family classification level. Cats and dogs are both from the Order: Carnivora and share common ancestors, reflecting their evolutionary relationships.
Step-by-step explanation:
Two different species of reptile would be most closely related if they share the same level of family in the biological classification system. In the case of cats and dogs, they are part of the same group beginning with the Order: Carnivora level. Cats and dogs share common characteristics and are considered to be more closely related as you move down the hierarchy of classification levels from domain to species. They are both within the Mammalia class, but belong to different families.
It's important to understand that the taxonomic classification system reflects evolutionary relationships, implying that all members of a taxon should have a common ancestor and be more closely related to each other than to members of other taxa. The species level is the most specific category in the classification system, highlighting the unique characteristics and lineage of an organism.