The basis of European Union began with the signing of—

The correct answer is Option 1: Maastricht Treaty.

While the process of European integration began earlier, the Maastricht Treaty (signed in 1992 and effective in 1993) is the definitive legal instrument that formally established the European Union (EU). It expanded the cooperation of the previous European Economic Community into a political union based on three pillars: the European Communities, Common Foreign and Security Policy, and Justice and Home Affairs.

• Treaty of Paris (1951): Created the European Coal and Steel Community (ECSC), the precursor but not the Union itself.
• Treaty of Rome (1957): Established the European Economic Community (EEC), focusing primarily on economic integration.
• Treaty of Lisbon (2007): Amended the existing treaties to modernize the EU's structure but did not "begin" the Union.

Thus, the Maastricht Treaty is recognized as the foundational document that transitioned the project from a purely economic entity into the modern European Union.