The correct answer is: d. Carassius is a group of different organisms with similar structures, while auratus is a smaller group of memebers that can only breed with another auratus.
Carassius represents the name of genus. In binomial nomenclature it is the generic name shared by the group of close relatives. In this case, Carassius is a genus in the family of ray-finned fishes.
Auratus represents the species of the fish and it is specific unique epithet within the genus.