I voted for the 1st answer which about35% of participants voted. I think it is more formal than the 2nd answer
Oops... Try the third: "for whom you've built the website."
'whom' is correct - (try saying you built it for him/them, which tells you it should be whom) - and it's bad form to end a sentence with a preposition (for), so number 3 is correct.
Actually, the sentence would read better without the 'about'!
The sentence is poorly written, but "whom . . . for" is the best of a poor selection..
I think third one should be correct