Answer is: an alpha particle.
Nuclear reaction: ¹⁶O + p⁺
→ ¹³N + α (alpha particle).
Alpha decay is radioactive decay in which an atomic
nucleus emits an alpha particle (helium nucleus) and transforms
into an atom with an atomic number that is reduced by
two and mass number that is reduced by four.
When oxygen-16 gain one proton, atomic mass is 17, but when lose alpha particle
atomic mass reduces by four to 13.
