The correct option is A Remarkable increase in the brain size.
Human evolution is characterised by a number of morphological, developmental, physiological, and behavioural changes that have taken place since the split between the last common ancestor of humans and chimpanzees. The most significant of these adaptations are as followed:
1. Bipedalism
2. Increased brain size
3. Lengthened ontogeny (gestation and infancy)
4. Decreased sexual dimorphism