Which directive was implemented to improve operational transparency in the EU's financial markets?

The directive implemented to improve operational transparency in the EU's financial markets is MiFID, which stands for the Markets in Financial Instruments Directive. MiFID was designed to create a more integrated and competitive financial market in the European Union by improving the transparency of financial transactions, enhancing investor protection, and ensuring the integrity of the market. It introduced requirements for firms operating within the EU to provide better information to their clients, ensuring that they are aware of the risks involved in trading financial instruments. This framework not only promotes fairness and efficiency in the financial market but also fosters greater competition among investment firms.

The other choices refer to different regulatory frameworks. Basel I and Basel II are primarily focused on banking capital adequacy and risk management rather than directly on operational transparency in financial markets. Sarbanes-Oxley is a US law aimed at protecting investors from fraudulent accounting practices, which does not specifically address the operational transparency of EU markets. This makes MiFID the most relevant directive for the question about improving operational transparency in the context of the EU's financial markets.