The correct option is
D Total intensity of the spherical wave over the spherical surface centered at the source remains constant at all times
When the travelling wave is coming from a very far-off source, the wavefronts are planar. For a plane wavefront, amplitude remains constant, therefore intensity remains constant.
![](https://df0b18phdhzpx.cloudfront.net/ckeditor_assets/pictures/1047170/original_6.png)
When the source of a travelling wave is linear, the wavefronts are cylindrical in shape. All the points are equidistant from the source.
In this case, amplitude of a wave,
A∝1√r.
Intensity of the wave,
I∝1r
When the source of a travelling wave is a point source, the wavefronts are spherical in shape. All the points are equidistant from the source.
In spherical wavefront,
Amplitude of a wave ,
A∝1r
Intensity of the wave,
I∝1r2
Since, all the points on the spherical surface are equidistant from a point source, total intensity is independent of time.
Hence, all options are correct.