The capacity in litres of a tank $0.5 m$ long, $12 cm$ wide and $60 mm$ deep is : 

$\text{ Length }=0.5\mathrm{m};\text{ width }=12\mathrm{cm}\times \frac{1\mathrm{m}}{100\mathrm{cm}}=0.12\mathrm{m}$

$\begin{array}{l}\text{ Depth }=60\mathrm{mm}\times \frac{1\mathrm{m}}{1000\mathrm{mm}}=0.06\mathrm{m}.\\ \begin{array}{ll}\therefore \text{ Capacity }& =\text{ Length }\times \text{ width }\times \text{ depth }\\ & =0.5\mathrm{m}\times 0.12\mathrm{m}\times 0.06\mathrm{m}\\ & =3.6\times {10}^{-3}{\mathrm{m}}^3=4\times {10}^{-3}{\mathrm{m}}^3=4\mathrm{L}\\ & \left[\therefore 1\mathrm{L}={10}^{-3}{\mathrm{m}}^3\right]\end{array}\end{array}$

In case of multiplication, the result is given in minimum number of significant figures

(see chapter, $2$) (i.e., $1$ as in $0.5$). So, $3.6$ is taken as $4.$ So, the correct answer is $\left(a\right).$