Answer
$^{12}_{6}C$
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:
$246+A=254+(4\times1)\implies A=254+4-246=12$
As the sum of charges must be equal:
$96+Z=102+(4\times0)$ or $Z=102-96=6$
The symbol of the element with atomic number 6 is $C$.
Therefore, the missing reactant is $\,^{12}_{6}C$.
