Which of the following is an effect of adverse participant reactions in a study?

Adverse participant reactions in a study often lead to decreased retention rates. This occurs because when participants experience negative feelings, discomfort, or dissatisfaction related to the study's procedures or their experiences within the study, they may choose to drop out or may be less likely to continue participating. This reaction can compromise the overall validity and reliability of the study's findings, as a lower retention rate can lead to a less representative sample and potential biases in the data collected.

In contrast, other options such as improved data quality, enhanced sample diversity, and increased participant engagement typically represent positive outcomes for studies, rather than effects stemming from adverse reactions. These latter aspects are generally seen in studies where participants are satisfied and feel positive about their involvement, which is not the case when adverse reactions are prevalent.