Step 1: Cauliflower's curd, like broccoli's head, is the apical meristem proliferated into branched, undeveloped floral shoots.
Step 2: It isn't a modified stem (compare kohlrabi) or modified leaves (leaves only surround it).
Step 3: Because it is a mass of unopened floral branches rather than individual flowers, modified flowers is too narrow a description.
Step 4: The correct term is modified inflorescence.