Answer: A) Cooing
Step-by-step explanation:
Cooing stage is a term that is used while development of baby or infant is taking place. Infants tend to produce vowel type sounds , gurgling sound etc to depict the language development. Example of cooing can be sound like"ahh".
According to the question,Paul is at the cooing stage where he is displaying his language development through sounds like"ooh" and "aah" while playing with truck toy.
Other options are incorrect because babbling is the talking meaninglessly. Murmuring is speaking quietly and humming is continuous and buzzing sound.Thus, the correct option is option(A).