When Multiple Currencies is enabled, Salesforce updates exchange rates across records to reflect current values, but closed opportunities remain unaffected. This behavior preserves the historical accuracy of closed deals, ensuring that revenue reports are based on the exchange rate at the time the opportunity was closed.
Exchange Rate Management: By not retroactively updating closed opportunities, Salesforce maintains the integrity of historical financial data, which is essential for accurate reporting.
Consistency in Financial Reporting: Current exchange rates apply to open records, while closed records retain their original converted amounts, balancing accuracy with historical consistency.
Option A (all objects supporting reporting) is inaccurate, and Option C (ability to disable) is incorrect as Multiple Currencies cannot be disabled once enabled. See Salesforce Currency Management documentation for more details.