Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
There is no way to see who did this. If you didn't do it, then it is did by someone who wanted to install unauthorized applications like on it like spyware on it. There are several ways to unroot it.
First method is by using Super SU app.
Note : Backup all your important files from your phone to your computer before unrooting it.
The second way is by flashing/installing a fresh software on it from your computer using Odin application. Let me know if the first way doesn't work, so I will send you instructions.