Driving to work one day, rosemary suspects that motorists talking on their cell phones process less information relevant to the task of driving than do other drivers. this hypothesis would most likely be tested by a psychologist.

cognitive psychologist

Explanation:

According to my research on studies conducted by various psychologists, I can say that based on the information provided within the question this hypothesis would most likely be tested by a cognitive psychologist. This is because this area of psychology focuses on studying the mental processes such as "attention, language use, memory, perception, problem solving, creativity, and thinking". Since Rosemary needs to test attention, perception, and thinking, then we can say this will be tested by a cognitive psychologist.

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