Six-month-old chester is sitting in his car seat and repeatedly saying "baabaa." chester is best described as

Chester would be described as a highly developed baby normally babbling starts around 9 months of age

The answer to the question, would be, that Chester is a babbler and he is in perfect synch with his stage of development, linguistically speaking.

Explanation:

Babbling, starts to appear almost from the first two months of life, and the sound is simply a reaction from the child to other sounds. However, around 2 months of age, babbling becomes more clear, and the apparition of vocalization of vowel sounds appears. Around the sixth month, the child must already be babbling by using double-syllable sounds. In the case of Chester, since he is doing exactly just that, using the double-syllable babbling sound, he is perfectly in-tune with his developmental stage.