Final answer:
A fixed exchange rate regime in which the monetary authority holds foreign exchange reserves backing 100% of its domestic currency issuance is known as a currency board.
Step-by-step explanation:
A fixed exchange rate regime in which the monetary authority is legally required to hold foreign exchange reserves backing 100% of its domestic currency issuance is best described as a currency board. In a currency board, the central bank or monetary authority holds foreign currency reserves equal to the amount of domestic currency in circulation. This ensures a fixed exchange rate and helps stabilize the country's currency.