The hypothesis is that baby talk is not just used for infants but is also a style of communication used towards any non-verbal listener, including pets. To support this hypothesis, we need evidence that shows a tendency to use baby talk characteristics (higher pitch, slower tempo) when addressing non-verbal individuals.
The finding that would most directly support this hypothesis is:
(Choice A) People tend to raise their pitch and slow their tempo when speaking to adult humans who are non-verbal.
This choice demonstrates that baby talk characteristics are employed not only with infants but also with non-verbal adults, thereby supporting the researchers' hypothesis that the usage of baby talk is based on the listener's non-verbal status rather than their age.