Which of the following scenarios is an example of a trade sanction? a. the united kingdom places a cap on immigration. b. australia hosts the summer olympics. c. the united states purchases foreign oil to compensate for the country's limited supply. d. the eu refuses to import diamonds from sierra leone due to human rights violations.

The correct answer is D. The EU refuses to import diamonds from Sierra Leone due to human rights violations.
This is a classical example of a trade sanction. One country that does not keep it self to some given standards, be it UN standards, EU standards, standards set between two countries, the other country, alliance or union will sanction the trade and hit directly on the economy of the country, thus forcing it to back off and respect the standards/rules.