= Solution
There are \b[two diagrams]: the $t$-channel <Z boson> exchange from the <weak neutral current>, and a crossed $u$-channel <W boson> exchange from the <weak charged current>. The charged-current vertices pair the incoming <neutrino> $k$ with the outgoing <Electron> $p'$ and the incoming <Electron> $p$ with the outgoing <neutrino> $k'$. The exchanged momentum has square $u=(k-p')^2$. With charge flow from the first vertex to the second, the exchanged boson is $W^+$. The corresponding panels show both topologies; no photon or Higgs diagram survives the stated <neutrino> assumptions.
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