= Solution
For an incoming $\mu^-$, a putative $e^-\mu^-\to\nu_e\nu_\mu$ process would change total <electric charge> from $-2$ to zero. It is forbidden: both negative <charged leptons> would have to emit a $W^-$, and a single exchanged charged boson cannot connect those two emission vertices. Ordinary <Elastic scattering>[elastic] $e^-\mu^-\to e^-\mu^-$ scattering remains possible through $t$-channel <photon> or $Z$ exchange, shown in the figure, but this is not the <Charged current>[charged-current] reaction of part d. The <Neutral current>[neutral-current] weak contribution is distinct from the electromagnetic one.
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