Final answer:
100 hydrogen stations are already in operation in Europe, which is leading the way in hydrogen infrastructure to promote cleaner energy sources for vehicles, contrary to Antarctica which is covered by an ice sheet.
Step-by-step explanation:
The correct answer to complete the sentence "100 hydrogen stations already in operation in ______" is A) Europe. Europe has seen significant growth in the development of hydrogen infrastructure aimed at promoting cleaner energy sources for transportation.
While other continents are also involved in the development of hydrogen fuel stations, Europe has been a leader in this initiative, particularly in countries like Germany, UK, and Norway.
There are various regions around the world that could potentially have hydrogen stations.
However, given the options provided and considering the fact that Antarctica is mostly covered with an ice sheet and would not have such infrastructure, along with the limited information regarding hydrogen stations in specific continents, Europe is the most likely answer.