Hi there. 

As far as my knowledge goes, the city in which you have been residing for the last 10 years will be your domicile. 

If you have given Class X and XII boards from Delhi then I'd assume that you have been living in Delhi from the last 10 years, hence making Delhi your domicile. 

Please comment your exact situation and feel free to post any further questions here. 

Hope this helps. 

Good luck!