Final answer:
The downside not associated with a within-subject single-blind experiment is (D) Participant bias.
Step-by-step explanation:
The downside of a within-subject single-blind experiment that is not represented in the options provided is (D) Participant bias. In a within-subject design, all participants receive each treatment, including the control, but are not aware of the sequence. This structure could introduce order effects, where the sequence in which treatments are administered affects the outcome. However, participant bias is typically a concern when participants know which group they are in and tailor their behavior to meet expectations. Since participants are blinded, participant bias is less of an issue. Demand characteristics (A) can affect results if participants guess the study's purpose and change their behavior accordingly. Experimenter bias (B) could occur if the researcher influences the study's outcome based on expectations. The within-subject design allows the same individuals to experience all treatments, which makes it easier to control for lurking variables and participant variables since each individual serves as their control.