(a) Paper notes
Modern currency includes paper notes and coins, authorised by the government, unlike old metallic coins of gold, silver, or copper.
Source: Modern Forms of Money, Chapter 3
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The passage clearly states: "Modern forms of money include currency — paper notes and coins… modern currency is not made of precious metal such as gold, silver and copper." Gold, silver, and copper coins belong to an older phase of money. The examiner expects you to pick paper notes as the correct option representing modern currency.