Dear Hardj

As you mentioned in the question about the way you had done for the particular question in the OMR sheet is highly wrong method, which may lead to negative marking and OMR challenge is done when you're challenging the response that OMR checking machine ( didn't check it or checked it wrongly ) has registered from your OMR answer sheet.

Candidates are advised to fill the circle completely while marking the answer. Sometimes, the machine cannot read the answers which are not marked properly, thus it will be considered as ‘not responded’ or ‘wrongly marked’ .

In your situation, there are two possibilities-

1. If the OMR machine scanned both the bubbles which means negative marking and

2. If one bubble is scanned, it will considered as positive marking (if answer found to be correct) or negative marking.

You should consider the above mentioned possibilities before going to any decision (challenge it or not) as it will cost you Rs.1000 for each question and money will not be refunded even the challenge is accepted . You can visit the below link for challenging

https://ntaneet.nic.in/Ntaneet/online/CandidateLogin.aspx