Mark (✓) against the correct answer:

(b) $40 \mathrm{~cm}$

Let the length of the parallel sides be $5 x \mathrm{~cm}$ and $3 x \mathrm{~cm}$, respectively.

$^{2}$ $A$ rea of the trapezium $=\left\{\frac{1}{2} \times(5 x+3 x) \times 12\right\} \mathrm{cm}$

$=\left(\frac{1}{2} \times 8 x \times 12\right) \mathrm{cm}^{2}=48 \times \mathrm{cm}^{2}$

But, the area of the trapezium is $384 \mathrm{~cm}^{2}$.

$48 x=384$

$\Rightarrow x=\frac{384}{48}=8$

$L$ onger side $=5 \mathrm{x}=5 \times 8=40 \mathrm{~cm}$