Please provide your PAN (Permanent Account Number) - It's a Preferable ID. If you don't have PAN, then you can provide AADHAAR Number.
If you don't have PAN and AADHAAR, then you can provide any one of the following unique identification number
03-Tax Identification number
04-Passport number
05-Elector's photo identity number
06-Driving License number
07-Ration card number
*** Please do NOT fill junk data or NA as this defeats the purpose***