Dr. Neelam Sanjiva Reddy was elected President of India unopposed.
Option 1, Dr. Rajendra Prasad, was indeed the first President of India, but he was elected through a secret ballot and not unopposed.
Option 2, Dr. S. Radhakrishnan, served as the second President of India, but he too was elected through an election process where he competed against other candidates.
Option 4, K.R. Narayanan, served as the 10th President of India, but he was also elected through a voting process and faced opposition from other candidates.
Therefore, the correct answer is option 3, Dr. Neelam Sanjiva Reddy, as he is the only one among the given options who was elected as President of India unopposed.