In Addition to Charge and Baryon Number, Another Conserved Quantity $X$

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 $X$ in the following reaction? $\mu^{-+} p \rightarrow n+X$

A) $\gamma$
B) $\mathrm{v}_{\mu}$
C) $\pi^{0}$
D) $\mathrm{e}^{-}$
E) $\mathrm{v}_{\mathrm{e}}$