Correct Answer - Option 4 : 0.0125 N/m

__Concept:__

Surface tension on a liquid droplet (Spherical droplet of water): Pressure intensity inside the droplet:

\(p = \frac{{4\sigma }}{d}\)

Surface tension on a hollow bubble (Soap bubble): Pressure intensity inside the droplet:

\(p = \frac{{8\sigma }}{d}\)

where σ is the surface tension of the liquid.

__Calculation:__

__Given:__

d = 40 mm, ΔP = 2.5 N/m2

\({\rm{Δ }}P = \frac{{8\sigma}}{d}\)

\(\sigma = \frac{{Δ P\times d}}{8 } =\frac{{40\times 0.0025}}{8 }= 0.0125\) N/m.