Đáp án đúng: D
Giải chi tiết:\(\begin{array}{l}\,f\left( x \right)\,\,\, = 3{x^2} - 2x - {x^4} - 2{x^2} - 4{x^4} + 6\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \left( { - {x^4} - 4{x^4}} \right) + \left( {3{x^2} - 2{x^2}} \right) - 2x + 6\\\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \, - 5{x^4} + {x^2} - 2x + 6\\g\left( x \right) = - {x^3} - 5{x^4} + 2{x^2} + 2{x^3} - 3 + {x^2}\\\,\,\,\,\,\,\,\,\,\,\, = - 5{x^4} + \,\left( { - {x^3} + 2{x^3}} \right) + \left( {2{x^2} + {x^2}} \right) - 3\\\,\,\,\,\,\,\,\,\,\,\, = \, - 5{x^4} + {x^3} + 3{x^2} - 3\end{array}\)
Chọn D