The type of pollination in which genetically different pollen grains fall on the stigma is known as:

Xenogamy

The correct answer is Option (3) → Xenogamy

Xenogamy: Transfer of pollen grains from anther to the stigma of a different plant .This is the only type of pollination which during pollination brings genetically different types of pollen grains to the stigma.

Autogamy: Autogamy refers to the transfer of pollen grains from the anther to the stigma of the same flower. It occurs within a single flower, and genetically, it involves the self-pollination of the plant. In autogamy, the pollen grains and stigma of the same flower come into contact, leading to fertilization with genetically identical pollen.

Geitonogamy: Geitonogamy involves the transfer of pollen grains from the anther to the stigma of another flower of the same plant. While functionally it is a form of cross-pollination since it involves a pollinating agent, genetically it is similar to autogamy because the pollen grains come from the same plant. Geitonogamy does not introduce genetically different pollen to the stigma.

Self-pollination is a type of pollination in which pollen from the anther of a flower is transferred to the stigma of the same flower or to another flower on the same plant.