Kylie is investigating parent-adolescent conflict in her master's thesis. She videotapes three groups of boys interacting with their mothers: a group of 11-year-olds, a group of 14-year-olds, and a group of 17-year-olds. Since she is comparing different age groups at the same time, Kylie is using a(n) research design.

The answer is a cross-sectional study.

Explanation:

A cross-sectional study observes a group of people with similar characteristics, except one, in a specific point of time. In this example, the aim is to understand how the age difference affects the interaction with the participants' mothers.

A key element of cross-sectional studies is that the studied variable remains constant, as the observation is done at the same time; unlike longitudinal studies, in which the variable may change because it is carried out over a long period of time.