Final answer:
Dogs, cats, and humans share common ancestors as indicated by their structural similarities, specifically in upper forelimbs, and evolutionary traits such as hair and amniotic eggs.
Step-by-step explanation:
Regarding the organisms dog, cat, and human, what is true is that these species all share structural similarities due to a common ancestor. The structural similarities, especially in the upper forelimbs as homologous structures, are evidence of evolutionary relationships.
While their limb structures have diverged due to different evolutionary pressures and functions, the underlying bone structure remains notably similar, indicating that these organisms inherited these traits from a common forebearer. In the case of dogs and cats, they fall under the same broader taxonomic groups due to sharing more recent common ancestors compared to their relationship with humans. Additionally, the fact that dogs, cats, and humans have hair and share the trait of having an amniotic egg at some point in their evolutionary history further supports the idea of a common ancestry.