Final answer:
The transition period in language development starts with variegated babbling and ends when the child says their first word, marking the beginning of active participation in verbal communication.
Step-by-step explanation:
The transition period in language development begins with the onset of variegated babbling and closes when the child produces their first word. This stage indicates a shift from the repetitive babble to more complex sounds that begin to resemble more closely the patterns of adult speech. Eventually, this stage culminates in the momentous achievement of a child's first meaningful word, symbolizing their initial active participation in verbal communication.