The answer key of JEE main conducted in January 2019 was released by NTA (National Testing Agency) on 14th January. 

Now NTA has also provided the link for challenging the answer key.

Let's suppose a student has marked option A for question 1. But in the answer key released by NTA the answer of that question 1 is mentioned as option B. Now, student is very sure of his/her answer that option A is only the correct answer of question 1. So the student can go to their respective link and by following steps can challenge the answer of that particular question 1 by saying that Option A is correct and not Option B. Challenging per answer will cost a student Rs. 1000. Certain notes or PDFs can also be submitted to support that Option A is only correct for Question 1.

If they find out that Option A was correct as mentioned by the student then Rs. 1000 will be refunded. 

Challenging a answer of any question wouldn't change the answer of student but it is for the students who feel that some other option will be correct instead of that answer mentioned by NTA in answer key. 

If you still have any queries feel free to ask in the comment section down below.