The stimulus tells us that babies who can hear and have hearing parents who expose them to speech begin to babble at a certain age as a precursor to speaking. We also know that deaf babies with deaf parents who communicate with them and with each other by signing begin to babble at the same age; this specific type of babbling consists of the deaf babies making repetitive hand gestures that constitute, within the language system of signs, the analogue of repeated syllables in speech.
The information provided in the stimulus can be used as evidence against answer choice (B). Since the stimulus tells us that deaf babies with deaf parents begin to make repetitive hand gestures that constitute the analogue of (i.e. something that is comparable to) repeated syllables in speech, they clearly do not depend on speech oriented vocal activity.
Thus, seeing as these deaf babies are developing language competency with gestures, as opposed to spoken words, it cannot be true that "the development of language competency in babies depends primarily on the physical maturation of the vocal tract, a process that requires speech oriented vocal activity."
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