Explanation:

A longitudinal study is a research design that involves repeated observations of the same variables i.e. people over a short or long period of time.
It is a type of observational study, although they can be structured as longitudinal randomized experiments. 
The huge disadvantage of any longitudinal study is the time, as it typically takes a substantial time to collect the data that is required.
Example: The 1970 British Cohort Study, which has collected data on the lives of 17,000 Brits since their birth in 1970, is one of the most well-known longitudinal studies.
Therefore, Longitudinal studies would be most appropriate to use in order to measure the changed opinions of consumers repeatedly.