Chọn phương án C.

\(\begin{aligned}
\dfrac{3x}{4-x^2}\geq1\Leftrightarrow&\dfrac{3x}{4-x^2}-1\geq0\\
\Leftrightarrow&\dfrac{3x-\left(4-x^2\right)}{4-x^2}\geq0\\
\Leftrightarrow&\dfrac{x^2+3x-4}{4-x^2}\geq0.
\end{aligned}\)

• \(x^2+3x-4=0\Leftrightarrow\left[\begin{array}{l}x=1\\ x=-4.\end{array}\right.\)
• \(4-x^2=0\Leftrightarrow\left[\begin{array}{l}x=2\\ x=-2.\end{array}\right.\)

Suy ra \(S=[-4;-2)\cup[1;2)\).