To determine when a cause-and-effect conclusion about the relationship between two variables in an experiment has been reached, the most relevant element among the options provided is random selection of participants.
Random selection helps ensure that the sample is representative of the population, minimizes bias, and increases the validity of the conclusions drawn from the experiment. This is crucial for establishing a causal relationship because it helps rule out confounding variables that could affect the outcome.
The other options—an international study, a small sample size, and a low-cost study design—do not inherently provide the same level of confidence in establishing a cause-and-effect relationship.