What type of bias occurs when a researcher unintentionally influences the responses of subjects in a study?

The correct answer, which identifies the type of bias that occurs when a researcher unintentionally influences the responses of subjects, is experimenter bias. This bias arises when the researcher’s expectations, beliefs, or behaviors inadvertently affect the outcome of a study. For instance, if a researcher demonstrates subtle cues, body language, or leading questions, they might alter how participants respond, leading to results that reflect the researcher's unintended influence rather than the participants' true opinions or behaviors.

Understanding this concept is crucial in ensuring the integrity of research findings. To minimize experimenter bias, researchers often implement blinding techniques where either the participants or the experimenters, or both, are unaware of critical aspects of the study, thereby fostering a more objective measurement of the dependent variables.

The other options represent different scenarios related to research but do not pertain specifically to the researcher’s influence over subjects. Instrumentation relates to the reliability and validity of measurement tools, the Hawthorne effect concerns changes in behavior when participants know they are being observed, and statistical regression deals with the phenomenon where values tend to move towards the mean over time during repeated measures.