Dear Arunima, Candidates can challenge the answer through the official website after the release of answer key. The objections raised by candidates against the wrong answer in NEET 2019 will be reviewed by expert committee and if they find objections are true then revised and final answer key will be released. The score of candidates will be calculated based on final answer key. So, if your challenge is incorrect then you won't lose any marks but yeah for surely you will lose your money.

All the best. 

Feel free to ask for any query.