Based on the information given, the thing that should be done to keep the exchange rate at $0.12 per dirham is to sell foreign currencies to get more dirham.
It should be noted that since the non-official supply and demand of dirham is pegged at $0.15/ Dirham and the actual rate is $0.12/dirham, it implies that the Dirham is devaluated.
The authority will believe that due to devaluation, net export will rise and this will increase the income of the country.
Lastly, the private investors and speculators will not invest because the currency has lost its value compared to the dollars.
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