Hi Madhukar, 

               I suggest you to cross check with the solutions for each question as the answer key is mostly correct 85%. But if you still have any discrepancy you can challenge the answer key. You will be required to pay Rs. 1000 per question to be challenged. Go to the official website of NTA and click on the answer key option. In the home page of the answer key window you will have an option "Challenge answer key". Then the answer key challenging portal will appear. Choose the questions and provide your claimed answer. Note that you need to check your challenges before you pay as they will be finalized before the payment portal. Then you can proceed to pay and finalize. If your claim is justified then the official final answer key will be changed accordingly and released. But I still suggest you to research on your claims before challenging. All the best!!