Which among these was the transitional stage between the apes and the humans?

Apes and humans both belong to the family of hominids. Humans are hominids because they are bipedal, have a large brain, and communicate using language. 

Australopithecus ramidus is the most ape-like hominid ancestor who walked upright on its hind limbs. It is thus considered the transitional stage between humans and apes.

The fossil of this hominid dates back to 4.4 million years and is the oldest and most primitive of all hominid fossils.