The characteristic shared by humans, wallabies, and trout is the notochord.
All three belong to the phylum Chordata, which is characterized by the presence of a notochord at some stage of their development. Humans and wallabies are mammals (subphylum Vertebrata) and have a notochord during their embryonic development, while trout, as fish, also exhibit this characteristic in their early development stages.
The other options (four limbs, mammary glands, and placenta) do not apply to all three groups:
- Four limbs: Only humans and wallabies have four limbs.
- Mammary glands: Only mammals, like humans and wallabies, have mammary glands.
- Placenta: Only eutherian (placental) mammals like humans and wallabies have a placenta.