Use mass–energy equivalence

The mass defect $\Delta m$ is related to binding energy $E$ by $E = \Delta m \, c^2$ So, $\Delta m = \frac{E}{c^2}$

Substitute the given values

Given: $E = 18 \times 10^8\ \text{J}$ and $c = 3 \times 10^8\ \text{m/s}$

Therefore, $\Delta m = \frac{18 \times 10^8}{(3 \times 10^8)^2}$

Simplify

$\Delta m = \frac{18 \times 10^8}{9 \times 10^{16}}$ $\Delta m = 2 \times 10^{-8}\ \text{kg}$

Convert into micrograms

We know: $1\ \mu\text{g} = 10^{-9}\ \text{kg}$

Hence, $2 \times 10^{-8}\ \text{kg} = \frac{2 \times 10^{-8}}{10^{-9}}\ \mu\text{g} = 20\ \mu\text{g}$

Match with the options

$\boxed{20\ \mu\text{g}}$

So the correct option is A.