How are a baby's early repertoire of responses primarily exercised?

The correct answer highlights the nature of a baby's early responses and indicates that these responses are primarily exercised spontaneously and without planned engagement. Infants are naturally inclined to explore their environment through instinctive reactions rather than through structured or formal activities.

In their early development, babies engage with their surroundings and caregivers through reflexive responses—such as crying, cooing, or making facial expressions—often occurring in moments that are not dictated by any formal structure or adult-led scenarios. This randomness allows for exploration and the creation of neural pathways as babies interact with various stimuli around them.

Structured activities are less characteristic of early infant development as they do not align with the innate, instinctual ways babies learn and communicate. Sleep, while critical for overall development, does not facilitate active responses; rather, it is a passive state. Social settings do play a significant role in a child's growth, yet they primarily engage with their environments through unstructured interactions rather than pre-set scenarios.

Understanding these early, spontaneous responses helps caregivers and educators recognize the intrinsic ways infants learn and adapt during their crucial developmental stages.