What type of research uses pre-existing groups without manipulating the independent variable?

The correct answer pertains to research that utilizes pre-existing groups and does not involve any manipulation of the independent variable, making it a form of observational study. In this case, naturalistic observation focuses on observing subjects in their natural environment, providing insights into behaviors or phenomena without any intervention on the part of the researcher. This method allows researchers to collect data on how groups behave in real-world settings rather than under controlled experimental conditions.

In contrast to naturalistic observation, longitudinal studies involve tracking the same individuals over time to observe changes or developments, which does manipulate the time frame but not the independent variable. Cross-sectional studies gather data from different groups at one point in time, which can compare various groups but still do not manipulate variables. Experimental designs are structured around manipulating an independent variable to observe the effects on a dependent variable, which directly contradicts the main characteristic of using pre-existing groups without manipulation.