Here, maximum stress $=10^9 \mathrm{~Pa}, \mathrm{~h}=3 \mathrm{~km}=3 \times 10^3 \mathrm{~m}$; $P$ (water) $=10^3 \mathrm{~kg} / \mathrm{m}^3$ and $\mathrm{g}=9.8 \mathrm{~m} / \mathrm{s}^2$. The structure will be suitable for putting upon top of an oil well provided the pressure exerted by sea water is less than maximum stress it can bear.
Pressure due to sea water, $\mathrm{P}=\mathrm{h} \rho \mathrm{g}$ $=3 \times 10^3 \times 10^3 \times 9.8=2.94 \times 10^7 \mathrm{~Pa}$
Since, the pressure of sea water is less than the maximum stress of $10^9 \mathrm{~Pa}$, the structure will be suitable for putting upon tap of the oil well.