Let the particle be X.
On summing the mass number on left hand side we get 253+4=257 units.
On right hand side we have 256 units only. So, we should have 1 unit mass number on X element.
On summing the atomic number on left hand side we get 99+2=101 units
On right hand side we have 101 units of atomic number. So, we should have 0 unit atomic number on X element.
So 10X=10n i.e. neutron.