Answer:
d. An experiment, because the students had control over which cereals would be selected for the study.
Step-by-step explanation:
In the context, the students are conducting an experiment because the students are selecting the cereals for which they noting down the information or data. In this the students are not just observing the sample but are actually taking the readings and notes down the sizes, names and prices of each sixth cereal from the row. Thus it is an experiment.
Hence the correct option is (d).