The transfer of pollen grains from one plant to the stigma of a different plant is known as: 

Xenogamy

The correct answer is Option (2) → 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.

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.

Cleistogamous flower : Cleistogamous flowers are those that self-pollinate without opening. Some plants such as Viola (common pansy), Oxalis, and Commelina produce two types of flowers – chasmogamous flowers which are similar to flowers of other species with exposed anthers and stigma, and cleistogamous flowers which do not open at all.