Final answer:
Australopithecus africanus is a gracile hominin characterized by a pronounced facial projection, small teeth, and a bipedal yet slender build with robust arms, indicative of both ground and arboreal activities.
Step-by-step explanation:
Gracile australopithecines like Australopithecus africanus are characterized by a more pronounced projection of the face (prognathism), smaller teeth and jaws, and a lack of a sagittal crest which is a feature found in their robust counterparts. Australopithecus africanus exhibits morphological traits that somewhat resemble modern humans but also retain some primitive features. For instance, while A. africanus was bipedal with a slender build, its brain was larger than that of A. afarensis, at about 500 cubic centimeters, yet this is still smaller than modern human brains. They also had robust arm bones and may have spent considerable time in trees, indicating a degree of arboreal activity.