What type of research involves studying naturally occurring relationships among variables?

Correlational research is the correct answer because it focuses on identifying and analyzing the relationships between two or more variables without manipulating them. This type of research seeks to determine whether a change in one variable might be related to or associated with a change in another variable, providing insight into their natural interactions. Unlike experimental research, which involves controlled conditions and the manipulation of independent variables to observe effects on dependent variables, correlational research is conducted in real-world settings where variables exist as they naturally do.

In the context of the other types of research mentioned, a pre-post study involves measuring variables before and after a treatment or intervention, which aims to assess the effectiveness of that intervention rather than examining relationships among variables. A non-equivalent groups study compares outcomes between groups that are not randomly assigned, usually in situations where random assignment is impractical. While these types of studies can also provide valuable insights, they do not primarily focus on the naturally occurring relationships among variables like correlational research does.