Final answer:
The psychologist should use observational methods and physiological measures to study whether babies prefer country music to rock music, as these provide both behavior observation and objective physiological data.
Step-by-step explanation:
The psychologist interested in determining whether babies prefer country music to rock music should opt for a combination of observation and physiological measures. Observational methods allow the psychologist to watch the babies in a natural or structured setting to see how they react to different types of music, capturing behavior in their natural context, something particularly important in developmental psychology. Physiological measures, on the other hand, could provide objective data on the babies' responses, such as changes in heart rate or brain activity, which can indicate preference or recognition. Integration of these two methods would provide a comprehensive assessment that accounts for both observable behaviors and internal physical states. Surveys and self-report measures would not be reliable for babies, as they cannot express their preferences verbally. Interviews with parents could offer insights but would be subjective and not directly reflect the baby's preference. Case studies focus on individual cases rather than a larger sample and are thus less generalizable, while retrospective analysis is not applicable to babies who have just begun to experience the world and have limited past experiences to analyze.