Dear Abhishek, you haven't mentioned the course that you are interested in. Neverthless, you can apply for the desired course at DU with your 12th from NIOS. Your admission will be based on the fulfillment of eligibility criteria predesigned for that particular course that you have chosen. In other words, if you meet the eligibility criteria of the course that you are applying for, your 12th from NIOS shouldn't be a problem because NIOS is a recognised body.