Placentation of plants refers to the arrangement of ovules within the ovary of a flower. The ovules are attached to the placenta, which is a structure within the ovary. The arrangement of the ovules on the placenta is important in determining the type of fruit that will form after fertilization. The different types of placentation include axile, free central, parietal, basal, and laminar, which are based on the position and attachment of the ovules on the placenta. Therefore, the correct answer is: "arrangement of ovules in the ovary".