Looks like a Eurasian Crag Martin to me - in Europe I’d have no hesitation but I’m not sure what the other options are here apart from a Dusky (which has darker underparts with concolourous utcs and a paler throat as well as being smaller than this appears to be.)
Given the number of views but no other opinions being proffered here either as an alternative to my suggestion or confirmation, is leaving me wondering what I am missing if this is not a rupestris?
Eurasian Crag Martin - note how light changes in the link below of a 1w bird with impression of underwing going from very contrasty between underwing coverts and the flight feathers, to low contrast as in the OP image where the underwing looks overall quite dark.
Are these all the same birds that you’ve added? # 2 and maybe #4 look like different birds with less contrasting coverts and paler brown with smaler less distinct spots? The vent also looks paler and less contrasting than the bird in image #1
Re the new images: Pale Crag Martin doesn’t occur in India (which has darker coverts restricted to carpel area) so maybe #2 at least is better for Dusky given the lack of contrast to the coverts and rest of underwing - it’s very hard judging plumage tones in such strong light which can have a tendency of ‘flattening’ contrasts or burning out colour tones. If an underexposed image has been lightened, this too can show produce ‘noisy’ and low contrast results.
I'm reminded of a time when I couldn't decide whether I was looking at Crag or Dusky Crag, because they would look like one then the other depending on how the light hit them :-O. And of course it's even easier to not know what's going on in digital images.