= Solution
\b[Allowed through a t-channel <W boson>]. Along one fermion line the <up quark> becomes a <down quark> by emitting a virtual $W^+$; along the other the <charm antiquark> absorbs it and becomes a <strange antiquark>. The middle panel shows this <weak charged current> exchange. Both <electric charge> and <baryon number> are conserved at each vertex, and the relevant <CKM matrix> entries are nonzero.
This is the unique physical-particle <tree-level Feynman diagram>. A neutral exchange would require a <flavor-changing neutral current> on each line, absent at tree level in the <Standard Model>. An annihilation into a neutral boson would likewise require an off-diagonal up-type neutral current. The different generations do not forbid the charged-current graph.
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