[tex]3x\sqrt[3]{648x^4y^8}\\\\=3x\sqrt[3]{216\cdot3x^{3+1}y^{3+3+2}\\\\=3x\sqrt[3]{216}\cdot3x^3x^2y^3y^2y^2}\qquad|\text{use}\ \sqrt{ab}=\sqrt{a}\cdot\sqrt{b}\\\\3x\sqrt[3]{216}\cdot\sqrt[3]{x^3}\cdot\sqrt[3]{y^3}\cdot\sqrt[3]{y^3}\cdot\sqrt[3]3xy^2}\quuad|\text{use}\ \sqrt[3]{a^3}=a\\\\=3x(6)(x)(y)(y)\sqrt[3]{3xy^2}\\\\=18x^2y^2\sqrt[3]{3xy^2}\to\boxed{B.}[/tex]
