Six-month-old grace's father is delighted when he comes into her room one morning and she says, "da da da." her alternation of consonant and vowel sounds is called:

Her alternation of consonant and vowel sounds is called babbling.

Your options were cooing, babbling, telegraphy, and holophrases. Cooing produces a sound, but it's a sound of a murmur rather than actual alternations of consonants and vowels such as da da da. Telegraphy is far too advanced for a child, and holophrases include entire words such as food to denote Give me food.