Step 1: Many Rubiaceae members, such as species used as examples in this family, bear a capsule as their fruit type, formed from a multicarpellary syncarpous ovary that splits open at maturity.
Step 2: Caryopsis is characteristic of grasses (Poaceae), follicle is typical of families like Apocynaceae, and pome belongs to Rosaceae, so none of these three fit Rubiaceae.