The correct answer is A. Vertical study.
A vertical study, also known as a longitudinal study, is a type of research study that follows a group of people over a period of time. This type of study is often used to track changes in a population over time, such as changes in height, weight, or health.
In the case of the question, the statement “Nine year olds are taller than seven year olds” is an example of a reference drawn from a vertical study. This is because the statement is based on the observation of a group of people (children) over a period of time (from age 7 to age 9).
The other options are incorrect because they do not describe the type of study that was conducted. A cross-sectional study is a type of research study that collects data from a group of people at a single point in time. A time series study is a type of research study that collects data from a group of people over multiple points in time. An experimental study is a type of research study that involves the manipulation of one or more variables in order to observe the effect on another variable.