when u 239 emits a beta particle the nucleus left behind has how many protons.

when uranium -239 emits a beta particle the nucleus formed has  one more protons, because beta decay  increases the number of protons in an atom by one, since uranium-239 has atomic number of 92, the subsequent atom will have an atomic number of 93.

²³⁹ ₉₂U → ²³⁹₉₃Np  + ⁰_-1e