For sound waves, once a wave has been initiated in whatever medium, it's frequency and wavelength should not change.
'Wave crest velocity' - do you mean the wave height, - it's amplitude?
That will decrease as the wave disperses from it's point of origin, it will do that in any medium.
A medium which is more viscous will absorb energy in the wave more quickly (more viscous = more friction), so the wave amplitude will therefore diminish more quickly.
I'm pretty sure the same principle applies with EM waves, other than noting that EM waves can propagate without any medium.
You can see on the linked page that velocity is affected by medium. So the question is, which of (ii) or (iii) is not?
Normally frequency stays the same.
Wavelength is the distance covered by the wave in one period, so that would clearly be impacted if the velocity changed.
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