Bhuvan, to be eligible for NEET, you should have completed your 12th, passed in all subjects and secure a minimum aggregate of 50% in Physics, Chemistry and Biology. It is 50% for General Category, 45% for PWD and 40% for OBC,SC,ST. Now if you belong to OBC Category, the Category you belong to should be also listed in the Central OBC list. Or else you will be considered as General Category and so you have to secure a minimum aggregate of 50% in Physics, Chemistry and Biology. This 50% is inclusive of the Theory and Practical marks scored in Physics, Chemistry and Biology collectively. You needn't score 50% in Physics, 50% in Chemistry and 50% in Biology separately. You need an aggregate of 50% or 40% depending on your Category as an aggregate in Physics, Chemistry and Biology. And along with Physics, Chemistry and Biology, you should also have studied English as one of the core subjects in your 12th. Now regarding your question, you cannot have Biology from NIOS as on demand exam. You have to take up Physics, Chemistry and Biology from NIOS. Biology as an additional subject is considered only if you studied it along with your other subjects in your 11th and 12th. But in your case, as that is not the fact, you have to appear for Physics, Chemsitry and Biology from NIOS to be eligible for NEET.