= Solution
Only $t$-channel <Z boson> exchange contributes at leading order. A <W boson> would connect $\nu_\mu$ to $\mu^-$, rather than to $e^-$, and therefore does not supply this elastic channel. With the assumptions and normalization of part (i),
$$
\boxed{\sigma_{\mathrm{ii}}=\sigma_*\left[(x-\tfrac12)^2+\frac{x^2}{3}\right].}
$$
For the illustrative value $x=0.231$, the dimensionless factors in parts (i) and (ii) are $0.5521$ and $0.09015$. The electron-flavor reaction is therefore about $6.1$ times as large in this limit. This uses the <weak flavour selection in neutrino-electron scattering>, not a hypothetical flavor-changing neutral vertex.
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