In a dioecious flowering plant, which types of pollination are preven…

The correct answer is Option (3) → Autogamy and geitonogamy

Pollination is of three kinds. Autogamy is transfer of pollen to the stigma of the same flower. Geitonogamy is transfer to another flower of the same plant. Xenogamy is transfer to a flower of a different plant. In a dioecious species each plant bears flowers of only one sex, so a plant is either male or female. No flower has both anthers and stigma, and no plant has a flower of the other sex. So autogamy and geitonogamy are both ruled out, and only xenogamy can take place.

Option (1) Cleistogamy and xenogamy — Incorrect. Cleistogamous flowers are bisexual flowers that never open, so they can only self-pollinate. A dioecious plant does not have them, but xenogamy is not prevented. It is the only pollination possible in such a plant.

Option (2) Geitonogamy and xenogamy — Incorrect. Geitonogamy is indeed prevented, because no other flower on the same plant carries the opposite sex. Xenogamy, though, is the one type that still happens.

Option (3) Autogamy and geitonogamy — Correct. Unisexual flowers rule out autogamy, and separate male and female plants rule out geitonogamy. Papaya is the book’s example of dioecy.

Option (4) Autogamy and xenogamy — Incorrect. Autogamy is prevented, but xenogamy is not. Pollen must travel from a male plant to a female plant, which is xenogamy.

NCERT Extract:
Pollination – “… Another device to prevent self-pollination is the production of unisexual flowers. If both male and female flowers are present on the same plant such as castor and maize (monoecious), it prevents autogamy but not geitonogamy. In several species such as papaya, male and female flowers are present on different plants, that is each plant is either male or female (dioecy). This condition prevents both autogamy and geitonogamy.”