Many capitulum inflorescences, several corymb inflorescences with capitulum inflorescences as the end nodes of each corymb inflorescence, or one compound corymb inflorescence with capitulum inflorescences as the end nodes?
A flowering plant's inflorescence is the grouping of flower clusters on the plant. Inflorescences are categorized based on the architecture of the arrangement of flowers within them.
As members of the sunflower or daisy family, Asteraceae, Sonchus has it's flowers compressed into tight heads called capitula. A sunflower might casually appear to be a single flower, but close examination will reveal that it is made of many (dozens in the case of Sonchus) smaller flowers. In the photo above, each yellow petal, including those in the interior of the disc, originates from a separate flower. As such, the inflorescence of Sonchus is a capitulum.
However, Asteraceae is an extremely diverse family, and the arrangement of capitula can be an important character in classifying and identifying these plants. Because of this, the same inflorescence terms applied to a group of flowers can also be applied to a group of capitula. In Sonchus one can (often) call the arrangement of capitula a corymb*. One definition of corymb is "a flat-topped or round-topped inflorescence, racemose, but with the lower pedicels longer than the upper".
So, strictly, the inflorescence of Sonchus olearaceus is a capitulum, and the capitula are arranged in (approximate) corymbs.
*The first resource I found listed Sochus as "corymbiform to subumbelliform" which means, roughly, ranging from a corymb (but maybe not exactly) to not-quite-an-umbel.