Answer: Babbling
At around 3-4 months, children start the babbling period, which is characterized by the production of sounds that approach the sounds of adult speech and whose frequency is maximum between 9 and 12 months. During this period, the sounds produced already have an upward and downward intonation pattern, however they are not intelligible until seven or eight months, when parents begin to identify babbling with requests, hunger, surprise, etc.
The role of this period in the acquisition process is not clear; according to Menyuk (1970, p.79) the purposes of babbling are "to explore the possibilities of the vocal mechanism, to feel pleasure in the vocal performance, to learn to control the output of the mechanism and thus, to repeat certain sequences. In a later stage, babbling has the function of drawing the parents' attention and communicating the child's emotional state.