Explanation:

It is the international currency in which the highest faith is shown and is needed by every economy. The strongest currency of the world is one which has a high level of liquidity, i.e. people are easily willing to sell or buy it due to the high confidence shown in it. Basically, the economy with the highest as well as highly diversified exports that are compulsive imports for other countries (as of high-level technology, defence products, life-saving medicines and petroleum products) will also create high demand for its currency in the world and become the hard currency. It is always scarce. Up to the Second World War, the best hard currency was the Pound Sterling (E) of the UK, but soon it was replaced by the US Dollar. Some of the hard currencies are Yen, Euro and Sterling Pound