What does the term fitness mean when biologists are discussing evolution?

Australopithecus robustus was likely the longest-surviving species of australopithecine in south africa. it had: a. large molars, a big face, and a sagittal crest. b. a large body, large teeth, and a sagittal crest. c. a big brain, big teeth, and a big face. d. a big face, large teeth, and a large body