Answer
$\,^{59}Fe\rightarrow\text{beta particle}+\,^{59}Co$
Work Step by Step
Let the missing reactant be $\,^{A}_{Z}X$.
The emission of a beta particle results in a unit increase in atomic number but the mass number remains constant.
Therefore, $A=59$
$Z$ of $Co$=$Z+1$ or $Z=27-1=26$
$X$ is the symbol of the element with an atomic number of 26. That is, $X=Fe$.
The missing reactant is $\,^{59}Fe$.
