Please explain experts,..,

The correct option is (3).
Xenogamy is a mode of pollination which involves the transfer of pollen grain from the flower of one plant to the stigma of the flower of another plant. Since cleistogamous flowers do not open i.e they are completely closed flowers, so the pollen grains fall on their own stigma which prevents cross-pollination or xenogamy and hence only self-pollination occurs in these flowers.

Hope this information clears your doubt.
Regards.