Natural pairings defines coupling of data sets or sample is possible to take place in a particular condition naturally.These samples depend on each other for matching.
According to the question ,cholesterol level of 90 men is being assessed prior and after treatment takes place.Thus,same 90 men are going through drug testing in natural pair form and showing sample dependency before and afterwards of treatment.Thus, further analysis is based upon natural paired data.
Other options are incorrect because sample in the question are not independent due to the link between them and they do persist natural pairing with each other.Thus, the correct option is option(D).
