**Solution**:

According to the question, $l=3 x, b=2 x$ and $h=x$

$\therefore$ Total S.A.

$=2(3 x \times 2 x+2 x \times x+3 x \times x)$

$\Rightarrow 352=2\left(6 x^{2}+2 x^{2}+3 x^{2}\right)$

$\Rightarrow 352=2 \times 11 x^{2}$

$\Rightarrow 352=22 x^{2}$

$\therefore x=\sqrt{\frac{352}{22}}=\sqrt{16}=4 \mathrm{~cm}$

$\therefore l=3 x=3 \times 4=12$

**So, the correct option is (D)**