Six West European countries (France, Germany, Italy, Belgium, the Netherlands, and Luxembourg) ended up pooling the production of coal and steel- two key industries for waging war - under a common supranational authority in 1951. This experience triggered economic integration among the members and it would later be followed by political integration in the early 1990s. Over time the number of member states has reached to 28 and regional integration in Europe has evolved into a sui generis regional polity – the European Union (EU). Spain was not among the first six countries. The correct answer is D.