Final answer:
The finding that would most directly support the researchers' hypothesis is that people tend to speak to other verbal adults with a lower pitch and faster tempo.
Step-by-step explanation:
The finding that would most directly support the researchers' hypothesis is option 2) People tend to speak to other verbal adults with a lower pitch and faster tempo. If baby talk is used when addressing any listener that is non-verbal, then it would be reasonable to expect that people would use a different speaking style when communicating with verbal adults. By speaking to other verbal adults with a lower pitch and faster tempo, it would indicate that the use of higher pitch and slower tempo is specifically directed towards non-verbal individuals, supporting the researchers' hypothesis.