In which type of research design do participants not know whether they are in the experimental or control group?

The correct answer is that participants do not know whether they are in the experimental or control group in a single-blind study. In such studies, the participants are unaware of the specific details regarding their assignment, which helps reduce bias that may result from their expectations about the treatment they receive. For instance, a participant who believes they are receiving an active treatment (experimental group) might behave differently than one who knows they are receiving a placebo (control group).

In this design, the focus is on how the participants' ignorance of their group assignment influences the study's results. This type of blinding helps to ensure that the outcomes are not influenced by participants’ preconceived notions or psychological responses to the treatment.

The other options indicate different research methodologies that do not describe the blinding of participants. In a double-blind study, neither the participants nor the researchers know who is in the experimental or control group, which further minimizes bias but involves an additional layer of blinding. Replication refers to repeating a study to see if the results are consistent and is not directly related to participant awareness. A field study involves observing subjects in their natural environment but does not specifically address participant knowledge about group assignments.