Solution

Following are the three strategies that a bisexual chasmogamous flower can evolve to prevent self-pollination (autogamy):
In many flowers, pollen release and stigma receptivity are not synchronized. Either the pollen is released much before the maturity of stigma or stigma matures much before the release of pollen.
In some flowers, anthers and stigma are placed at different places so that pollen grains from the same flower cannot reach the stigma.
Some flowers follow self-incompatibility between pollen and stigma. This is a genetically mediated process which prevents autogamy in these flowers.