Dioecy in some plants can prevent ________.

Dioecy is the condition in which male and female flowers are borne on different plants, meaning each individual plant is either male or female (e.g., papaya).

Since a single plant does not possess both male and female reproductive organs, self-pollination cannot occur. Therefore, this condition prevents both autogamy (pollination within the same flower) and geitonogamy (pollination between different flowers of the same plant).

It promotes xenogamy (cross-pollination), which is the transfer of pollen grains from the anther of one plant to the stigma of a different plant.

Answer: autogamy