We find the lepton numbers before and after the forbidden decay of the muon into an electron and a single neutrino. The electron and muon numbers are initially 0 and +1 respectively. If the neutrino is a muon neutrino, then the electron and muon numbers are +1 and +1 respectively at the end, which violates conservation of electron number. If the neutrino is not a muon neutrino, then the muon number at the end is , which violates conservation of muon number. So, in any case the decay would violate conservation of lepton number. Therefore, answer (E) is correct.