Final answer:
The group interaction between the single, pregnant teenager and the mother of three represents Yalom's curative factor of universality, which helps group members realize the commonality of their experiences.
Step-by-step explanation:
The scenario where a single, pregnant teenager in a parenting class expresses ambivalence toward her pregnancy and experiences guilt, which is then addressed by a mother of three sharing her similar feelings, illustrates one of Yalom's curative factors known as universality. This factor is defined by the recognition among group members that they are not alone in their experiences, thoughts, or feelings. In this context, the mother's admission validates the teenager's feelings, showing that such ambivalence is a common experience among pregnant women, thereby potentially alleviating the teenager's sense of isolation and guilt.