Yes you are eligible to do your btech. course if you have biology as a subject in class 12th along with PCM.At the time of admission in NIT you just have to pass your exam in all the 5 subjects PCM and english and a extra subject which yours in biology. For admission in IIT you have to get 75% marks if you are a CBSE students and Other State board students have to be in top 20 percentile .So ,don't worry and just focus on your 12th board.

All the best for future.