Final answer:
To test a hypothesis involving reinforcement predicting language dialects, an b. experimental study is the most suitable research design as it allows for variable manipulation and control groups.
Step-by-step explanation:
Dr. Smith wants to test a hypothesis that reinforcement of a dialect predicts dialects observed within the same speaking language region. To adequately test the cause-and-effect relationship inherent in this hypothesis, Dr. Smith should use an experimental study. This type of study allows for the manipulation of variables and the establishment of control groups, thereby enabling the researcher to observe the effects of reinforcement on dialects. The hypothesis suggests a direct influence, implying that an experimental approach is the most appropriate research design to determine if reinforcing a particular dialect will result in that dialect being more widely observed in the region.