Which of the following describes an "observational study"?

An observational study is defined as a type of research design in which researchers observe subjects in a natural environment without any interference or manipulation of variables or conditions. This approach allows researchers to gather data on participants' behaviors, outcomes, and interactions as they naturally occur, which can be particularly valuable for identifying associations and patterns that might exist in real-world situations.

In observational studies, the focus is on watching and recording what is happening without altering any factors. For instance, researchers may observe the dietary habits of a group of individuals over time to see how these habits relate to health outcomes, but they do not intervene or change what those individuals eat.

Other options may represent different types of research methodologies. For example, manipulating variables would occur in experimental studies, where researchers actively change elements to observe effects. Research conducted without human participants typically applies to animal studies or laboratory experiments, which doesn't align with the definition of observational studies involving human subjects. Lastly, relying solely on interviews does not encompass the breadth of data collection methods used in observational studies, which often include direct observation as a primary tool.