Developmental psychology treats growth as the joint product of what a child brings at birth and what the surroundings supply afterwards. The biological inheritance carried through genes fixes the range of possibilities, while family, school, nutrition, culture and companions decide how far within that range the child actually travels.

Woodworth compared the two to the length and breadth of a field, and most modern writers describe them as interacting rather than competing: an inherited potential stays unrealised without supporting surroundings, and no amount of stimulation can push growth beyond what the inherited structure allows. Together they account for every aspect of human growth.

The other choices are themselves outcomes of this pair. Reasoning power is largely an inherited capacity shaped by experience, a learned ability is acquired through training in a particular setting, and illness is only one of many environmental conditions that may slow growth down.

Heredity and environment option is correct.