Different people have different opinions. In my opinion, it's another male, who could have gotten Sarafina pregnant with Nala before he was kicked out by Mufasa. I have made a fanfiction on this theory, and if you want to read it, it's on fanfiction.net. It's called Nala's Quest.

Well we have to remeber that there still are other males in a pride but they all get chased out later in their life so its possiable that one of those male lions were Nala's father but i can see why you would think scar would be Nala's father because Sariphina(Nala's mother) used to have a crush on scar

Yeah, like Kh said, and many also think is just one lion not in pride(me).

It a bit depends how do you look things, more from human point of view she can't be Scars, because Simba would be her cousin. And Mufasa wasn't. So there must be other lion.
If you look it from lions point of view, Scar is strong possibility, because when male lion takes over pride, he kills other lions cubs, so there would be impossible for Nala to be daughter of any other lion. Also they protect prides from other males(Mostly one male is in pride, rarely two if pride is big). And females usually don't accept other lioness in pride. So all is left are Mufasa or Scar. And lions don't look at relationships.

No-one knows for sure who Nala's father is. Many think it is Scar, but most people make up their own character to be her father.