The correct option is
A Union of dissimilar parts
Adhesion in a flower occurs when the dissimilar floral parts unite with each other. For example, stamens can unite with petals to form epipetalous flowers, stamens can unite with perianth lobes to form epiphyllous flowers, stigma, style and stamens can unite to form gynostegium etc.
Hence, the correct answer is 'Union of dissimilar parts'