Answer
$^{4}_{2}He$
Work Step by Step
Let the missing reactant be $\,^{A}_{Z}X$ where $X$ is the symbol, $Z$ is the atomic number and $A$ is the mass number.
As the sum of masses on both sides must be equal:
$253+A=256+1\implies A=257-253=4$
As the sum of charges must be equal:
$99+Z=101+0$ or $Z=101-99=2$
The symbol of the element with atomic number 2 is $He$.
Therefore, the missing reactant is $\,^{4}_{2}He$.
