Let me break it down for you, starting with:
A. That was when I realized my decision was final; but there was no turning back. Now, at first glance, this seems like a pretty good answer, right? Wrong, you don't need that "but" there, doesn't it seem a bit out of place? Now, I'm not saying this isn't something you can do, but it's not the best answer choice.
B. That was when I realized my decision was final; however, there was no turning back. "However" is a word that you usually don't see people use a lot. Now, to me, as someone who likes lengthy writing, I would choose this answer choice, but since it asks for the best answer choice, this one isn't the correct one.
C. That was when I realized my decision was final; also there was no turning back. Anyone with a brain can tell that this is obviously a wrong answer; you wouldn't just say, "Also, there was no turning back." in the middle of a sentence, it doesn't fit there.
D. That was when I realized my decision was final, and there was no turning back. Read this aloud, you can tell that it sounds and slides off the tongue nicer than the other three sentences.
D is the correct answer.
