The type of pollination which brings genetically different types of pollen grains to the stigma is -

Xenogamy

The correct answer is Option (3) → Xenogamy 

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.

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. 

Polygamy means a type of mating system in which an individual has more than one mate at the same time. Polygyny → One male mates with many females. (common in lions, red deer, etc. Polyandry → One female mates with many males. (seen in some birds like jacanas, honeybees).