Which of the following is a threat to internal validity?

Selection is indeed a threat to internal validity because it refers to the way participants are chosen for a study. If the method of selecting participants leads to groups that are not comparable at the start of an experiment, it can result in systematic differences that could affect the outcome of the study. For example, if one group consists of inherently healthier individuals while another group has a mix of health statuses, it becomes challenging to determine whether the observed effects are due to the intervention or the pre-existing differences in health among the groups.

In contrast, sampling generally pertains to the technique used to select individuals from a population, which influences external validity more than internal validity. Generalization refers to the applicability of the study results to wider populations, which addresses external validity rather than internal validity. Coherence refers to the logical consistency of the research findings and the connection of evidence, which is not directly classified as a threat to internal validity. Thus, selection stands out as a significant factor that can directly compromise the integrity of the experimental conditions and the conclusions drawn from a study.