The correct answer is - Coincidentally, my oldest sibling, Nate, just turned nineteen. It is quite easy to spot a subject - just ask the question who. Who just turned nineteen? - My oldest sibling. The first option is incorrect because the phrase is used as an indirect object. C is incorrect because my brother Nate is the subject, and my oldest sibling is the appositive. D is incorrect because Nate is the subject, and my oldest sibling is predicate nominative.
I will not write this for you when this is something pretty easy to do. You just need to do a tiny bit of research. You can do this. You are capable of doing this. I believe in you.