Which one of the following is a means of controlling for observer bias? a. Using unobtrusive observations. b. Waiting for the participants to become used to the observer. c. Making sure the observer does not know the study's hypotheses. d. Measuring physical traces of behavior rather than observing behavior directly.

c. Making sure the observer does not know the study's hypotheses.

Observer bias occurs when the observer's expectations or knowledge about the study influence their observations. To control for this bias, it is important to ensure that the observer is unaware of the study's hypotheses. This way, their observations are less likely to be influenced by their preconceived notions or expectations about the outcomes.