Device to discourage self-pollination or increase cross-pollination is
a. pollen release and stigma receptivity are not synchronised
b. anther and stigma placed at different position
c. same height of stamen and stigma

Flowering plants have developed many devices to discourage self-pollination. In some species, pollen release and stigma receptivity is non-synchronised, i.e. either the pollen is released before the stigma becomes receptive or stigma becomes receptive much before the release of pollen.
In other species, the anther and stigma are placed at different positions, so that the pollen cannot come in contact with the stigma of the same flower. Both these devices prevent autogamy (self-pollination) and increases cross-pollination.