The correct answer is A) Spine.
Humans share homologous structures with other vertebrates, and the spine is one of them. It is a common feature in vertebrate anatomy, representing a shared evolutionary origin among these species. Hair (B) is not present in all vertebrates, tails (C) are not present in fully grown humans, and gills (D) are not present in adult humans but are found in aquatic vertebrates during their developmental stages.