Final answer:
The Phi Delta Theta fraternity's official flower, described as having ray flowers and disk flowers arranged in spirals, is the purple chrysanthemum.
Step-by-step explanation:
The question "What is Phi Delt's flower?" refers to the flower associated with the Phi Delta Theta fraternity. The flower we're discussing here has ray flowers and disk flowers, as mentioned in the resources provided. The ray flowers are the outermost part of the flower, featuring large petals that extend radially. Within the center of these blossoms are numerous disk flowers organized into spirals. This description fits well with the flower known as the purple chrysanthemum, which is the official flower of Phi Delta Theta.