Final answer:
The adaptive radiation of australopithecines primarily focused on mastication, with particular evolutionary adaptations in teeth and jaw structures to process food efficiently, not on bipedalism, brain size increase, or body size increase.
Step-by-step explanation:
The adaptive radiation of the australopithecines after their split from the lineage that led to early Homo primarily focused on characteristic c. mastication rather than bipedalism, brain size increase, or body size increase. The australopithecines exhibited a number of evolutionary adaptations relating to their diet and caloric requirements, in particular changes to their teeth and jaw structures that were suited to process their food efficiently. This was a necessary evolution due to the changing climate and available resources.
While bipedalism was a key feature shared between Australopithecus and modern humans, it was already established by the time early hominins split from the lineage leading to Homo, which later saw increases in brain size and body size as key diagnostic features in the genus.