Final answer:
An experimental study involves two groups with different treatment variations to determine cause-and-effect relationships. This is different from observational studies, which cannot definitively establish causation.
Step-by-step explanation:
The type of research method that involves two groups receiving different variations of the treatment to understand cause-and-effect is the c) Experimental study. In an experimental study, researchers manipulate the independent variable to measure its effect on the dependent variable, comparing the results with a control group that does not receive the experimental treatment. This contrasts with observational studies, where researchers observe and measure variables without manipulation.
In an experimental study, such as testing whether regular aspirin intake reduces the risk of a heart attack, subjects are divided into two groups: one receives the treatment (aspirin) and the other receives a placebo. The outcomes are then compared to establish a causal relationship. On the other hand, observational studies can show correlation, but they cannot firmly establish causation due to the lack of control over variables.