Answer: C. experimental study.
Experimental studies are used to establish cause-and-effect relationships between variables by manipulating one variable (independent variable) and observing the effect on another variable (dependent variable) while controlling for other potential factors. Correlational studies examine the relationship between two variables but do not establish causality, descriptive studies describe a phenomenon without manipulating variables, and meta-analysis is a statistical method that combines the results of multiple studies to provide an overall summary.
Explanation: