Megha, the Supreme Court has passed an order stating that Aadhar Card is not mandatory for academic and entrance examination purposes. Apart from Aadhar Card as an NRI, as mentioned in the Information Bulletin at: https://ntaneet.nic.in/Ntaneet/ShowPdf. , you can also give  your Passport number. Ideally either Passport number or last 4 digits of your Aadhar card are allowed. If you do not have the Aadhar card yet, you can still upload your passport details. You still have time until the December 31st and so use the link: https://ntaneet.nic.in/Ntaneet/Registration/NewReg.aspx for registration.