$\begin{align}  & \text{Convert 12}{{\text{5}}_{x}}\text{ to decimal system} \\ & 1\times {{x}^{2}}+2\times {{x}^{1}}+5\times {{x}^{0}}=20 \\ & {{x}^{2}}+2x+5=20 \\ & {{x}^{2}}+2x-15=0 \\ & (x+5)(x-3)=0 \\ & x=-5\text{ or }x=3 \\ & \text{None of the option are correct, because in base 3, digit 5 cannot exist} \\\end{align}$