Self-pollination is prevented by ?

Hint: Self-pollination is seen in bisexual flowers. Plants have adopted several mechanisms to prevent self-pollination. It is both genetically induced and growth related. Sometimes the anther is more developed than the stigma or vice-versa. Morphological structure of the reproductive part also plays a vital role in preventing self-pollination. Continuous self-pollination may lead the offspring to become weaker and less susceptible to diseases.

Complete answer:
Self-pollination is a type of pollination in which pollen from one plant is transferred to the stigma of the same plant. There are two types of self-pollination. In autogamy pollen is transferred to the stigma of the same flower and in geitonogamy, pollen is transferred from the anther of one flower to the stigma of another flower on the same flowering plant.
Self-pollination is prevented by various mechanisms. When the flower of one plant possesses some mechanical barrier on its stigmatic surface, it may affect the self-pollination. This condition is called herkogamy. Self-pollination could also be prevented genetically. When a plant cell contains a male sterility homozygous gene in its nucleus or cytoplasm, the anther of that plant will lose its ability to transfer pollen to the stigma. This is known as genetically induced male sterility.
Heterostyly is the condition where the flowers of some plants have different lengths of stamens and styles so that self-pollination is prevented. When pollen and stigma of the flower mature at different times then self-pollination is not possible. This condition is called Dichogamy.

Note:
The advantage of self-pollination is that only one type of plant is required for the process. Since the plant will inherit its gene from the parent plant, beneficial qualities can be passed down to its offspring. Self-pollination does not produce new variants and thus less likelihood of improvement in the next generation.