The correct answer is D.
Trade sanctions are common mechanisms in international affairs. They are imposed by a country or group of countries that set a penalty on imported products from one or more other nations.
Sanctions aim to force the punished nations to change their behaviour or policies on a certain issue, by limiting their ability to trade with the nation that has imposed the sanction and, in turn, their economic growth opportunities. This is the case of the US and Sierra Leone. The US imposes the sanction aiming to force Sierra Leone to respect human rights in diamond production.
