In this question,

$$\begin{gathered} \Rightarrow 1246+\sqrt[4]{256}=\left(x^2+19\frac{1}{3}\right)\times 15 \\ \Rightarrow 1246+4=\left(x^2+\frac{58}{8}\right)\times 15 \\ \Rightarrow \frac{1250}{15}=x^2+\frac{58}{3} \\ \Rightarrow \frac{250}{3}-\frac{58}{3}=x^2 \\ \Rightarrow \frac{192}{3}=x^2 \\ \Rightarrow x=\sqrt{64}=8 \end{gathered}$$

Hence, Option D is correct.