= Solution
\b[Allowed through an s-channel <W boson>]. The initial <up quark> and <down antiquark> annihilate into a virtual $W^+$, which produces the final <charm quark> and <strange antiquark>. The two <weak charged current> vertices contain the nonzero <CKM matrix> elements $V_{ud}$ and $V_{cs}$, with <complex conjugations> determined by fermion-flow conventions.
The left panel shows the unique physical-particle <tree-level Feynman diagram>. A neutral exchanged <gauge boson> cannot connect these charged annihilation currents, and flavour-diagonal neutral vertices cannot turn an <up quark> into a <charm quark>. There is no additional elementary charged scalar in the minimal <Standard Model>.
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