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Determine which Galactic source class (supernova remnants, young massive star clusters, microquasars, the Galactic center) accelerates protons to at least 3 PeV, and whether the knee is a source maximum-rigidity feature ($E_{\\max}$ proportional to charge $Z$) or a propagation feature. Answer: an identified PeVatron class with an acceleration model reproducing the proton and helium knee positions.","problem_ref":"1b487bd2a57e219f95646aa3ba694016eb080e96dbda690e183584ccb77fd11c","references":"","settled_by":"Composition-resolved spectra of protons, helium and heavier groups up to 100 PeV showing rigidity-ordered breaks, plus spatial identification of PeV proton sources through hadronic gamma rays above 100 TeV or neutrinos.","status_note":"2025: LHAASO measured a proton-only knee at about 3 PeV after a spectral hardening, supporting a rigidity-dependent source cutoff; the source class is unidentified.","title":"What produces the cosmic-ray knee near 3 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