IQ has been used to predict academic success. But what about other factors?

According to an article by Angela L. Duckworth and Martin E.P. Seligman in the journalPsychological Science, self-discipline is even a better predictor of academic success than IQ.

One way to measure self-discipline is the $1-now or $2-later test (which the researchers call the Delay Choice Task) in which we see if a subject would decline to accept a dollar bill now when told they could wait a week and get two dollars. Highly self-disciplined adolescents outperformed their more impulsive peers on every academic-performance variable, including report card grades.

Which of the following is correct?

This study is an experiment, since it was based on a random sample.

This study is an observation study since the researchers did not assign the students to one of the self-discipline categories.

This study is an experiment, since each subject was classified into a self-discipline category ($1-now or $2-later) and to report card grades.

This study is a combination of both an experiment and an observational study.