An advantage of using longitudinal studies is that a. they are relatively inexpensive b. they take little time to conduct c. they generally have small amounts of data d. subject variation is often reduced

The correct answer is option D.

Explanation:

Longitudinal study is an example of an observational research in which observations or data collected or taken repeatedly from the same subject for period of a time. These studies can extend over years or even decades.

In these type of studies less number of subjects are studied and observed.This is because subjects under observation are observed for a longer duration of time. Hence, subject variation is often reduced