Which situation is an example of an observational study?

a.) having a randomly selected group of students take a pen and paper test and comparing the results with another randomly selected group of students who took the same test on the computer?

b.) polling voters about their favorite candidates in upcoming elections.

c.) recording the reaction time of participants in a study who were asked to press a button as soon as they see a certain picture on a screen.

d.) asking kids in a kindergarten classroom about the average number of pieces of fruit they eat each day.