Final answer:
The study population is best when it's a representative sample of the target population (C), using random sampling to ensure an equal chance of selection and maintaining the external validity of the study results.
Step-by-step explanation:
The determinants of external validity revolve around how well a study population represents the target population. The claim that the study population is always identical to the target population is incorrect; practical circumstances often mean researchers work with a smaller segment, or a sample, of the target population. The most accurate statement among the options given is that the study population should be a representative sample of the target population (C). This representative sample aims to reflect the characteristics of the target population to ensure applicable and generalizable results. When researchers employ random sampling, they increase the chances that the sample will be representative, as every member of the population has an equal chance of being selected.
Furthermore, the size of the sample can also influence external validity; too small a sample might not capture the diversity of the target population. Researchers often strive for a sample size that is large enough to reduce sampling error and increase reliability. Sampling bias, on the other hand, occurs when certain members of the population are more likely to be selected than others, which can detract from the external validity of a study.