A person with full discretionary powers to act on behalf of a country

Answer: Option C

One word substitute is Plenipotentiary.

Ambassador: an accredited diplomat sent by a state as its permanent representative in a foreign country.
Emissary: a person sent as a diplomatic representative on a special mission.
Plenipotentiary: having full power to take independent action.
Envoy: a messenger or representative, especially one on a diplomatic mission.