\(D=\dfrac{\sqrt{25a^4-150a^5+225a^6}}{\left(3a-1\right)^2}\)
\(=\dfrac{\sqrt{25a^4\left(1-6a+9a^2\right)}}{\left(3a-1\right)^2}\)
\(=\dfrac{5a^2}{\left(3a-1\right)^2}\cdot\left|3a-1\right|\)
\(=\pm\dfrac{5a^2}{3a-1}\)
\(a=\sqrt{x}+\dfrac{1}{3}\)
=>\(a^2=\left(\sqrt{x}+\dfrac{1}{3}\right)^2=x+\dfrac{2}{3}\sqrt{x}+\dfrac{1}{9}\)>0
\(3a-1=3\cdot\left(\sqrt{x}+\dfrac{1}{3}\right)-1=3\sqrt{x}\)>0
=>D>0
=>\(D=\dfrac{5a^2}{3a-1}=\dfrac{5\left(x+\dfrac{2}{3}\sqrt{x}+\dfrac{1}{9}\right)}{3\sqrt{x}}\)
\(=\dfrac{5\left(\sqrt{x}+\dfrac{1}{3}\right)^2}{3\sqrt{x}}\)
\(=\dfrac{5\cdot\dfrac{1}{9}\left(3\sqrt{x}+1\right)^2}{3\sqrt{x}}\)
=>Chọn B
