The study involving random assignment to treatment or control groups in the treatment for depression is an experimental study. It is designed to analyze the cause-and-effect relationship and includes the use of a placebo to control for participant expectations.
The option (C) is correct.
In the context of operant conditioning and the treatment for depression, the study described involves screening participants and randomly assigning them to either treatment or control groups. This is characteristic of an experimental study, where the main intent is to analyze the cause-and-effect relationship by manipulating the independent variable and measuring the effects on the dependent variable.
Such studies rely on the random assignment of the participants to control for extraneous variables and provide stronger evidence of the treatment's effectiveness, in contrast to observational studies where researchers do not apply any treatment.
Experimental studies often include placebos to maintain a blind study structure and ensure that the results are not influenced by participant expectations. Given this methodology, the correct answer to the question would be a) a controlled experiment.