Detailed Solution:
The correct answer is “Germany”.
The Ruhr area is one of the most important industrial regions in Germany and was historically significant for its coal and steel production.
The Ruhr coalfield is situated in the state of North Rhine-Westphalia, encompassing cities such as Essen, Duisburg, Bochum, and Dortmund. The coalfield covers an area of approximately 1,400 square kilometers, making it one of the largest coal reserves in Europe.
The coalfield played a vital role in the industrialization of Germany in the 19th and 20th centuries, contributing to the country's economic growth.