Final answer:
The scenario described is an experimental study where participants are randomly assigned to use different toothpaste brands, and this controlled comparison is aimed at determining the effect on cavity incidence. Experimental studies are capable of suggesting causation due to the manipulation of independent variables.
Step-by-step explanation:
The question you're asking refers to a scenario where participants are randomly assigned into two groups, one using Crest toothpaste and the other using Brand X for a year to compare the incidence of cavities. This type of research study is best described as a D) Experimental Study. In experimental studies, researchers manipulate one variable, known as the independent variable, and measure the effect on another variable, called the dependent variable. In this case, the independent variable is the brand of toothpaste (Crest vs. Brand X), and the dependent variable would be the number of cavities participants have at the end of the year.
Research methodologies that do not involve direct manipulation of the independent variable, such as naturalistic observation and correlational studies, are used when it is not ethical or practical to conduct an experiment. Observational studies measure naturally occurring phenomena without intervention, whereas correlational studies assess the strength and direction of the relationship between variables without implying causation. On the other hand, experimental studies like the one described can suggest causation, as they involve controlled comparisons with variables that researchers manipulate to observe the effects.