d
(d) For a travelling wave, the intensity of wave remainsconstant if it is a plane wave.
Intensity of wave is inversely proportional to the square of the distance from the source if the wave is spherical
\(\left( {I = \frac{P}{{4\pi {r^2}}}} \right)\)
Intensity of spherical wave on the spherical surface centred at source always remains same. Here total intensity means power \(P\).