To answer the question, "did the program work?" researchers rely on which form of evaluation research?

The correct answer is impact analysis

Impact analysis refers to research that evaluates how successful or effective a given program was in bringing about some type of desired result or change. For instance, let's assume that a high school decided to make homework submissions digital rather than through written submissions, in order to motivate students to do their homework more regularly. Following this, an impact analysis would be able to evaluate whether digitizing homework submissions did in fact increase the frequency of students' homework submissions.