Final answer:
The research described in the question is an example of a correlational study, where the researcher measures the brand of toothpaste and the number of fillings/cavities to examine the relationship between them.
Step-by-step explanation:
The research described in the question is an example of C) Correlational Study.
In a correlational study, the researcher measures two or more variables and examines the relationship between them without manipulating any variables.
In this case, the researcher is interested in the relationship between the brand of toothpaste (Crest or Brand X) and the number of fillings/cavities.
The researcher will collect data from participants about their toothpaste brand and their dental condition, and then analyze the correlation between the two variables.
For example, the researcher may find that participants who use Crest toothpaste tend to have fewer fillings/cavities compared to those who use Brand X toothpaste.
However, correlation does not imply causation, so it cannot be concluded that Crest toothpaste directly causes fewer fillings/cavities.
correct option C) Correlational Study.