Assertion: Autogamy is a transfer of pollen grains from an anther to the stigma of the same flower on the same plant.
Reason: Xenogamy is pollination between two flowers on different plants.

Both Assertion (A) and reason (R) are correct  but R is not the correct explanation of A.

In autogamy , pollination is achieved within the same flower of same plant .Although geitonogamy is functionally cross-pollination involving a pollinating agent, genetically it is similar to autogamy since the pollen grains come from the same plant. Xenogamy is the only type of pollination which during pollination brings genetically different types of pollen grains to the stigma.