A dioecious flowering plant prevents both:

Explanation

Dioecious plants are those plants in which male flowers and female flowers are borne on different plants. Therefore, they prevent both autogamy and geitonogamy. Autogamy refers to the fusion of two gametes that come from one individual. Geitonogamy is the fertilization of a flower by pollen from another flower on the same (or a genetically identical) plant. Xenogamy is the transfer of pollen grains from the anther to the stigma of a different plant. This is the only type of cross-pollination that during pollination brings genetically different types of pollen grains to the stigma. Cleistogamy means the formation of flowers that do not open, and thus the production of seeds is a result of autogamy.