I wouldn't pick any of these.

1.) The water buffalo theory is unrealistic. If that really had happened, his eye would've been poked out and at least a buffalo horn would never cause that kind of a scar.

2.) His father's name is not Ahadi in the movie universe. And even if we chose to think about that book universe version of Scar, Ahadi wasn't implied to be an abusive father. And I have no need to imagine he was. (Not that I cared for those books anyway.)

3.) And the rest of the options while otherwise ok, don't fit my logic either.

His birth name is NOT Taka in the movie universe, but I doubt it was meant to be Scar either, so, as he likely wanted to be called Scar after receiving the scar, and the movie didn't make any dramatic references to its origin, it likely came from something heroic or otherwise cool. And I doubt a fight with a brother or getting slashed by a girl would be cool in anyone's eyes.

I don't really know yet what exactly I personally like to think its story to be. Only that it was some heroic deed he did, perhaps in an effort to prove himself to his father.

This is exactly what happened! It says so in 'A Tale of Two Brothers.'

^Not entirely true.

Yes, it happened that way in the mentioned book, but it didn't happen that way in with the movie's Scar.

That novel wasn't created nor written by Disney at all but it is an original story by an outsider author, and has been downright denied by Scar's creators as in the film's makers, on the commentary track of the Laserdisc and DVD. (Through denying Kopa, who is the main character in that novel.)

I'm not sure, but I think either the Buffalo or Ahadi are possible choices.

bendaimmortal is right, the people said it was from a water buffalo but it looked like someone clawed him with their sharp claws when they got mad.

I watched the Taka Story in parts and I heard the entire thing kind of like a prequel to me.

Scar's dad was... a drinker. An a fiend... And one night he got more crazy as usual...
It was a joke, haha!

They've said it was a buffalo in the Lion King wiki, but I don't buy it. I think it was a claw. It was a clean cut, something a buffalo couldn't do.

Yes, my thiory

Far as the movie, it's unknown what his father's name is. Ahadi is a fanfic name for his father since those books were not written by Disney. I believe a rouge lion could possibly swiped him, and gave him the scar. It's really unknown how Scar got his scar over his eye.