The areas of two circles are in the ratio 9 : 4.

Let the the radii of the two circles be r and R, the circumferences of the circles be c and C and the areas of the two circles be a and A.
Now,

$\frac{a}{A}=\frac{9}{4}$

$\Rightarrow \frac{\pi r^{2}}{\pi R^{2}}=\left(\frac{3}{2}\right)^{2}$

$\Rightarrow \frac{r}{R}=\frac{3}{2}$

Now, the ratio between their circumferences is given by

$\frac{c}{C}=\frac{2 \pi r}{2 \pi R}$

$=\frac{r}{R}$

$=\frac{3}{2}$

Hence, the correct answer is option (a)