Which aspect of a research study does random assignment specifically aim to control?

Random assignment specifically aims to control the assignment of subjects to treatment groups within a research study. This technique is crucial because it helps ensure that each participant has an equal chance of being placed in any of the groups, whether they are receiving a treatment or a control condition. By doing so, random assignment minimizes the potential for bias in assigning participants, which could lead to systemic differences between groups that could confound the results. This process helps to enhance the internal validity of the study, allowing researchers to make more accurate inferences about the effects of the interventions being tested.

In the context of the other options, while the variety of instruments (A) and collection methods (D) are important aspects of the research design, they are not directly influenced by random assignment. The selection of subjects (B) refers to how participants are chosen to be part of the study in the first place, which is typically addressed through random sampling or other methods but does not involve random assignment, which occurs after subjects have already been selected.