Students ask every 5th person to board the train what their favorite color is. which type of study method is described in each situation? a. controlled experiment b. survey c. observational survey d. none of the above

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.