The following is the answer—the retyped sentence as per the directions in the "question".
_________________________________
In an effort to realize his dream, DUNCAN, an aspiring astronaut, spends hours studying to prepare for his college entrance exams.
_________________________________
Answer to the second part of the question: The noun to which the appositive refers in the above sentence is: "astronaut".
___________________________________
{Additional note: the "appositive" in the [rewritten] sentence is "Duncan". "Duncan" (name of student and prospective astronaut) is the "appositive"—in the form of an "appositive noun" [as opposed to an "appositive phrase".]}.
________________________________________
