A dioecious flowering plant prevents both Autogamy and xenogamy, Autogamy and geitonogamy, or Geitonogamy and xenogamy?

The correct option is C Autogamy and geitonogamy. Autogamy is pollination by transfer of pollen grains from the anther of a flower to the stigma of the same flower. Geitonogamy is pollination by transfer of pollen grains from the anther of a flower to the stigma of another flower of the same plant. A plant having unisexual flowers is said to be dioecious. Such a flower will neither exhibit autogamy nor geitonogamy. Thus the correct answer is option C.