Which of the following could be a potential effect of participants leaving a study?

The choice that indicates a reduction in sample size as a potential effect of participants leaving a study is accurate because when participants withdraw from a research study, it directly decreases the number of individuals representing the target population. This can lead to a smaller sample size, which can impact the study's power and the generalizability of the findings. A reduced sample size may limit the ability to detect significant effects or may introduce bias if the remaining participants differ systematically from those who left.

On the other hand, the other options do not reflect the implications of participant attrition accurately. For example, increased reliability of results would typically require a stable group of participants and would not likely result from losing participants. Improvement in overall data accuracy is unlikely when participants leave, as missing data can introduce uncertainty and complicate analyses. Lastly, enhancement of participant motivation is contrary to the idea that participants are leaving, as higher motivation usually correlates with retention rather than dropout. Hence, the most relevant effect of participants leaving is indeed the reduction in sample size.