Water is filled into a container with hexagonal cross section of side 6 cm. If the surface tension of water is 0.075Nm^-1 then the surface energy of water will be

The surface energy of water surface = surface area $\times$T

Thus the surface energy of water's surface = 

$=\left(6\times \frac{\sqrt{3}}{4}{\mathrm{a}}^2\right)\mathrm{T}$

$=\left(6\times \frac{\sqrt{3}}{4}\times {(6\times {10}^{-2})}^2\right)\times 0.07\mathrm{T}=7\times {10}^{-4}\mathrm{J}$

Hence the correct answer $7\times {10}^{-4}\mathrm{J}.$