Final answer:
International airspace begins 12 nautical miles off a nation's coastline, which is beyond the limit of a nation's territorial sea as per international law.
Step-by-step explanation:
The question of where international airspace begins is a matter of international law and is determined by conventions regarding territorial waters and airspace. According to the United Nations Convention on the Law of the Sea, a nation's territorial sea extends up to 12 nautical miles (NM) from its coastline. Beyond this limit lies the international airspace, where no single nation has jurisdiction. So, the correct answer to the question is c. 12 NM off a nation's coastline.