Superscript o subscript negative 1 e.
Explanation:
The nuclear reactions is of 2 types, one is nuclear fusion and the other one is nuclear fission.
Nuclear fusion is nothing but the combining of 2 nuclei with an emission of energy along with an electron, proton or beta particle.
Nuclear fission is the break down of a nucleus into 2 or more nuclei along with an electron, proton or beta particle.
And the reaction is,
₅₆B¹²⁴ ₅₅C¹²⁴ + ₋₁e⁰
So the blank was filled by means of a beta particle.