Flowers may be solitary, as in a zinnia or dahlia, or appear in clusters or a/an , as in a/an . g

The answer is "inflorescence, oak"

Explanation:

An inflorescence is a flower group, which are clustered nearby in a stalk, which is also known as the peduncle.  In lily, the individual flower was found, not quite as groups. However, the flowers are set as an inflorescence in the oak tree. It refers to a group or community of a flower cluster borne on a flower, which is traditional flowers of oak are called catkins, that's why 'inflorescence, oak' is the reaction.