Perhaps I should not have used the word “drop/ed”. Actually Obama didn't do it. His step father, Lolo Soetoro, adopted Obama and in doing so Obama became a citizen of Indonesia. Since Indonesia did not allow dual citizenship, Obama’s US citizenship (if he actually was born on US soil) would have been(given up;renounced;suspended;dropped; ?)
Obama as an adult could have gone through the process of legally reestablishing his US citizenship but no one has seen the documents showing that he did give up Indonesian citizenship and reinstate his US citizenship.