Answer: It is a helium nuclei.
Explanation: Alpha particles are produced when a larger nuclei decays into smaller nuclei in radioactive decay. Alpha particles have mass number and atomic number as 4 and 2 respectively.
General representation of alpha decay :
[tex]_Z^A\textrm{X}\rightarrow _{Z-2}^{A-4}Y+_2^4\textrm{He}[/tex]