= Solution
\b[Absent in the minimal Standard Model.] The single charged-lepton <Yukawa matrix> supplies both the <fermion mass matrix> and the Higgs coupling, so the same left/right rotations diagonalize both. There is no off-diagonal $h\tau\mu$ vertex. With massless neutrinos, the separate charged-lepton family numbers prevent this transition perturbatively. If <neutrino> masses and mixing are added, loop-induced <charged-lepton flavour violation> can occur but is extremely suppressed; a sizeable rate would require an additional source of flavour violation. Part (c) provides one such source.
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