Although the actual questions are missing from your ask, here's how you can tell.
If the study will be messed up (added bias) through interaction with the subjects the study should be observational. For example if a biologists wanted to study the eating habits of wild birds in the tropics, they would need to do an observational study. if they did a controlled experiment, the birds will change their eating habits based on human interaction, thus making the study invalid.
If the study requires specific measurements, close monitoring, or contact and interaction with the subjects the study should be a controlled experiment. For example, if a scientist wanted to test out a new drug, the health of patients would need to be closely monitored.
Observational Study: Why does a rolling ball slow down and How effective is a new toothpaste in reducing cavities.
Controlled experiment: How does the daytime temp affect the formation of dew, Which is a better absorber of heat and Can plants prepare food in artificial light
Explanation: i just took the test and these are the correct answer.
