The average mass of one$\mathrm{Mg}$ is$24.305 \mathrm{amu}$, which is not the actual mass of one Mg atom.
Three isotopes,  ${}^{24}Mg{,}^{25}Mg$, and  ${}^{26}Mg$ of $\mathrm{Mg}$ are found in nature.
 

Due to the existence of three isotopes of magnesium,  ${}^{24}Mg{,}^{25}Mg$ and  ${}^{26}Mg$, the average mass of one $\mathrm{Mg}$ atom is $24.305 \mathrm{amu}$, which is different from the actual mass of one $\mathrm{Mg}$ atom.
Hence, option A is correct.