Chọn phương án C.

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

Vậy \(S=\{0;3\}\).