The correct answer is: of.
Sanjay was sure of getting a first class in the examination. This means that he was confident that he would achieve the highest grade possible. He may have been sure because he had studied hard, or because he was a naturally gifted student. Whatever the reason, he was confident in his ability to succeed.
The other options are not as accurate. Sanjay was not “at” getting a first class, meaning that he was not in the process of getting it. He was also not “on” getting a first class, meaning that he was not focused on getting it. Finally, he was not “about” getting a first class, meaning that he was not thinking about getting it.
In conclusion, the correct answer is “of.” Sanjay was sure of getting a first class in the examination.