Sarah wants to conduct a study about differences in the levels of emotion of jealousy between men and women. She asks 400 male and female college graduates in the United States a series of questions about hypothetical scenarios of partner infidelity. What is Sarah's population?

Target Population : All the men & women in the world

Study Population : Men & women in Sarah's college, near her locality, in her city (in US)

Explanation:

Population is the entire set, consisting all members of a group (sharing a common similarity), subject to conducting a research study.

Target Population is the group, to whom a researcher wants to generalise its findings. It denotes whole universe of people selected as the objective research group. It's also called Theoretical population.

Study Population is the group that is accessible by researcher, for carrying out the study. It denotes actual sampling frame, from which researcher draws the sample. It's  also called Accessible Population.

Sample is a smaller subset of population group, drawn to conduct a research study & find inferences about population.

A study about differences in the levels of emotion of jealousy between men and women :

c. taught scientists many new things about the earth, but not other planets.

explanation:

the density would be a smaller number.