Answer:
d. employed individuals aged 25-29
Explanation:
"Population" in a research study is the comprehensive group that the experimenter or the researcher is interested in.
It is given that US Census Bureau, showed that percentage of the employed individual who are of age group 25 years to 29 years are college educated and is at all time high.
The research study focuses on the specific age group of individuals those who graduated form college or at least have a bachelor degree.
Thus the population of the research study those who studied in each of the four years are the employed individuals aged from 25-29.