Final answer:
The kingdoms of Castile and Aragon formed a double monarchy through the marriage of King Ferdinand of Aragon and Queen Isabella of Castile, maintaining separate governments while being ruled by the same monarchs.
Step-by-step explanation:
The kingdoms of Castile and Aragon are often referred to as a double monarchy because of their union through the political marriage between King Ferdinand of Aragon and Queen Isabella of Castile in 1469. Despite being united under a single monarch, each kingdom preserved its separate systems of governance, laws, administrations, and institutions. The integration of these kingdoms laid the groundwork for the modern nation of Spain, initiating a period known as its 'golden age', marked by significant religious, economic, and political developments. This unification through a strategic marriage, while still maintaining distinct governmental systems, is why the correct answer to the initial quiz question is D) They were united through a political marriage. It's important to note that even though they shared monarchs, Castile and Aragon did not lose their individual identities or independence immediately and continued to function as separate realms within a larger political framework.