Which prevents self pollination

Flowers of insect pollinating plants are….

Choose correct sentence for pollination.

$(I)$ Majority of flowering plants use a range of animals as pollinating agents.

$(II)$ Among the animals, insects particularly flies are the dominant biotic pollinating agents.

$(III)$ Majority of insect pollinating flowers are small and less colorful.

$(IV)$ The body of the animal gets a coating of pollen grains, which are generally sticky in animal pollinated flowers.

In nature, cleistogamous flowers are