have 4 rounds process was simple its one day process, 1st round is aptitude round based on simple apti and c programming questions 2nd round was technical test round, 3 is technical interview, and last is HR

I got a call from HR and she scheduled my first round with one of the Technical person from their organisation. The interview lasted for 1 hour and i answered all the questions including programm snippets and code. Multithreading , C++ basic polymorphism related questions and related to pointers are there. After the interview and answer all questions I'm sure about this round but the hr said you're not selected , there's no reason i found to reject. try to be specific with rejection as much you're specific while selecting candidate and ask for info.