Which is correct? if i didn't have a good book, i would get so bored, while i wait for my brother to finish t-ball practice. if i didn't have a good book, i would get so bored while i wait for my brother to finish t-ball practice.

I would say the second one- it sounds like a run on, but if you read them out loud, it's the one that sounds right. If you were to say the sentence, there's no sense in stopping and pausing your sentence after "bored". Honestly, they probably shouldn't have put the word "so" in either sentence because it doesn't flow well, but they probably did it that way to throw you off. The first one has an unnecessary pause, where the second one doesn't. 
Good luck!!

Sometimes when you read things in your head, you skip overall small details/words. when you read aloud, you hear those words and it’s easier to comprehend what you just read. saying things aloud also you remember them.