Which study described below qualifies as research involving human subjects?

The correct choice involves a cognitive psychologist studying mood effects on problem-solving. This qualifies as research involving human subjects because it directly examines how mood influences cognitive processes in people. The focus on human participants in the context of psychological research ensures that the study gathers data about their behavioral and mental responses, making it essential to comply with ethical guidelines and regulations regarding research with human subjects.

In contrast, the other options either do not involve human subjects at all or lack the necessary elements to classify as research on human subjects. For instance, a survey collecting demographic data could be done without direct contact with specific individuals, or the data might not pertain to human behavior directly but instead focus on statistics. A literature review of psychological theories involves analyzing existing research rather than conducting new research with human subjects, and an observational study on animal behavior does not involve humans, hence exempt from the regulations that govern human subjects research.