If the surface tension of soap solution is $\sigma$, what is the work done in blowing soap bubble of radius r?

Since a bubble has two surfaces in contact with air, the total surface area of the bubble $=2\times \left(4\pi r^2\right)=$ $8\pi r^2$.

Now work done = surface tension x increase in surface area $=\sigma \times \left(8\pi r^2-0\right)=8\pi r^2\sigma$
Hence the correct choice is (d).