Unisexuality of flowers prevents which type of pollination?

The correct answer is Option (3) → Autogamy but not geitonogamy.

Pollination has three types. 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. A unisexual flower has either stamens or a pistil, never both. So its own pollen can never reach its own stigma, and autogamy becomes impossible.

Option (1) Xenogamy but not geitonogamy. — Incorrect. Xenogamy is not blocked by unisexual flowers. Cross-pollination between plants is the very thing that this device encourages.

Option (2) Geitonogamy but not autogamy. — Incorrect. This reverses the two terms. Geitonogamy can still take place on a plant bearing both male and female flowers. It is autogamy that is prevented.

Option (3) Autogamy but not geitonogamy. — Correct. A unisexual flower cannot pollinate itself, so autogamy is prevented. On a monoecious plant, pollen from a male flower can still reach a female flower of the same plant, so geitonogamy is not prevented.

Option (4) Both autogamy and xenogamy. — Incorrect. Xenogamy is never prevented by unisexual flowers. Even dioecy, where male and female flowers are on different plants, blocks autogamy and geitonogamy, not 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.”