Learning means a relatively lasting change in behaviour brought about by experience. Such changes can be observed from the earliest days of life, when a baby begins to recognise the mother's voice, turns towards light and sound, and slowly adjusts its sucking, crying and grasping to the responses it receives.

Developmental psychology treats the first few years as the period of the fastest learning, because the habits of feeding, sleeping, speaking and responding to people are laid down then and become the base on which everything learnt later is built.

The two later stages named here cannot be the starting point. By the time a learner enters school a very large store of language and social behaviour has already been gathered, and the teenage years only refine and extend what is already there. Since learning plainly does begin at the earliest stage, the denial alternative is ruled out as well.

infancy option is correct.