Linked from
The 4 pages that link to Down quark, each with the reason it gives.
ProtonBroader topic: The proton contains one down quark alongside two up quarks.
PionRelated: Down quarks combine with up antiquarks to form negatively charged pions.
IsospinBroader topic: It forms the quark-level partner of the up quark under isospin rotations.
Up quarkRelated: Up and down quarks are the light flavors that make ordinary nucleons.