 Solution

The dominant interaction is strong. At hadron level, panel (iii) is an effective hadronic vertex for $\Delta p\pi$. The initial $uuu$ constituents rearrange into $uud$ and $u\bar{d}$ after creation of a $d\bar{d}$ pair:One original up quark enters the charged pion; the other two join the new down quark in the proton. Gluon-induced pair creation supplies the quark-flow diagram interpretation. This is nonperturbative hadronization: the effective hadronic vertex does not claim that one elementary perturbative quark graph calculates its width. Charge, baryon number and net quark flavor are conserved.