Flowering plants such as castor and maize have developed devices to d…

The correct answer is Option (1) → Male and female flowers present on the same plant which prevents autogamy but not geitonogamy.

Autogamy is pollination within the same flower. Geitonogamy is the transfer of pollen from the anther of one flower to the stigma of another flower of the same plant. Genetically it is like self-pollination.

(1) Male and female flowers present on the same plant which prevents autogamy but not geitonogamy. — Correct. Castor and maize are monoecious. Their flowers are unisexual, so a flower cannot pollinate itself. But pollen can still reach a female flower of the same plant.

(2) Bisexual flowers and show both autogamy and geitonogamy. — Incorrect. Castor and maize have unisexual flowers. Bisexual flowers that self-pollinate do not discourage self-pollination.

(3) Male and female flowers present on different plants which prevents both autogamy and geitonogamy. — Incorrect. This is dioecy, as in papaya. In castor and maize, both kinds of flowers are on the same plant.

(4) Cleistogamous flowers. — Incorrect. These flowers never open, so they always self-pollinate. This is the opposite of what the plants in the question do.

NCERT Extract:
Pollination – “… The third device to prevent inbreeding is self-incompatibility. This is a genetic mechanism and prevents self-pollen (from the same flower or other flowers of the same plant) from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil. 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.”