a. a 10-month-old baby
Babies communicate using sounds and gestures. In the first year of life, babies go from babbling to playing with sounds, copying sounds and putting sounds together. First words might start at around 12 months. Babies start understanding and responding to words in the first year of life.
Children acquire language through interaction - not only with their parents and other adults, but also with other children.
Therefore, option a is correct.
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