I think it's Letter B, survey since they are asking about people's opinion (their favorite color), and it is more on a descriptive research. It doesn't really matter that it's the fifth person to board the train or not.
It is not Letter A since a controlled experiment is an experimental research by nature and would specifically deal with scientific procedures.
I'm not so with the Observational Survey. Both are ways of gathering information, but a survey is completed by another person (meaning the person they asked) while the observational survey is done by the observer only.