Answer:
"Since I want to be accepted to an ivy league college".
Step-by-step explanation:
The given sentence is: Since I want to be accepted to an ivy league college, I plan to study hard during my high school years.
Now, a dependent clause is a clause that provides a sentence with additional information, however it cannot stand alone as a sentence.
Now, in the given sentence in the question, the depend lent clause will be;
Since I want to be accepted to an ivy league college.
This is because it tells us why you plan to study hard during your high school years. However, it cannot stand alone as a sentence.