Geitonogamy and autogamy both are prevented in which one of these?

The correct option is Papaya.

To understand why both autogamy and geitonogamy are prevented in papaya, let us break down the mechanisms of pollination and plant sexuality:
1. Autogamy: This is a type of self-pollination where pollen grains are transferred from the anther to the stigma of the same flower. For autogamy to occur, the flower must be bisexual (containing both male and female reproductive parts).
2. Geitonogamy: This is the transfer of pollen grains from the anther of one flower to the stigma of another flower on the same plant. While it is functionally cross-pollination (often requiring a pollinating agent), it is genetically similar to autogamy because the pollen grains come from the same genetic individual. Geitonogamy occurs in monoecious plants, which have separate male and female flowers on the same individual plant.

For both autogamy and geitonogamy to be prevented, a plant must be dioecious. In dioecious species, male and female flowers are borne on entirely separate individual plants. This means an individual plant is either exclusively male or exclusively female. Consequently, pollen must travel from a male plant to a female plant, making cross-pollination (xenogamy) the only possible mode of sexual reproduction.

Let us examine the options provided:
- Castor, Maize, and Cucumber: These plants are monoecious. Since male and female flowers are present on the same plant, autogamy (self-pollination within the same flower) is prevented, but geitonogamy (pollination between different flowers of the same plant) can still take place.
- Papaya: Papaya is a dioecious plant where male and female flowers are borne on different individual plants. Because of this complete separation of sexes at the organismal level, both autogamy and geitonogamy are completely prevented in papaya.