Answer: Naturalistic Observation
Explanation: Naturalistic Observation involves the study of a subject in their natural environment. The purpose of this study is to understand how the subject interacts with their environment naturally without the conditions of a laboratory. Another factor which may make this kind of study attractive is a limited budget.
For example, in Professor Carter's study, recreating a supermarket in a laboratory will not only be expensive but may also influence the subjects' choice of groceries and affect the result of the study.