In Addition to Charge and Baryon Number, Another Conserved Quantity $\mathrm{X}$

Multiple Choice

In addition to charge and baryon number, another conserved quantity in particle interactions is lepton number. The six leptons have lepton number +1 , and their antiparticles have lepton number -1 . The lepton numbers for each generation (electron, muon, tau) are separately conserved. For example, an electron has electron-lepton number +1 , while the electron antineutrino has electron-lepton number -1 . What is the missing particle $\mathrm{X}$ in the following decay? $\quad \pi^{+} \rightarrow X+v_{\mu}$

A) $\mu^{0}$
B) $\mu^{-}$
C) $\pi 0$
D) $\mu^{+}$
E) $\pi^{-}$