= Solution
\b[Allowed through a virtual <W boson>]. The <charm quark> emits a $W^+$ and becomes a <down quark> through the nonzero <CKM matrix> element $V_{cd}$. The virtual <W boson> produces a <positive muon> and a <muon neutrino> through the leptonic <weak charged current>. The right panel is the unique physical-particle <tree-level Feynman diagram>.
Its total <electric charge> is $-1/3+1=2/3$, equal to the initial charge, and its final <lepton number> is $-1+1=0$. The decay is <Cabibbo suppressed>, rather than forbidden. These diagram counts use <unitary gauge>: in covariant gauges charged unphysical <Goldstone bosons> can supply gauge-dependent pieces of the same physical amplitudes, not additional physical channels.
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