Sunflower is not a flower rather a bunch of flowers. In fact, what appears to be a single flower is in fact a group or bunch of very small flowers called an inflorescence. These flowers are aestived on a receptacle in a special arrangement. This type of inflorescence is called capitulum. Following are the key features of capitulum.
$$*$$ Receptacle is flattened.
$$*$$ Many small and sessile florets are present on the receptacle.
$$*$$ The youngest florets are at the centre while the oldest are at the periphery.
$$*$$ There are two kinds of florets in sunflower, viz. ray florets and disc florets.
$$*$$ Ray florets are on the rim of the receptacle. These are of yellow colour.
$$*$$ Disc florets are in the centre. They are bisexual and actinomorphic. The above features show that sunflower is not a flower but an inflorescence.