'Summit Diplomacy' Is a Term That Refers to Which One

'Summit diplomacy' is a term that refers to which one of the following?

A) The fact that the first European heads of states meeting after the Second World War was held in the European Alps.
B) Diplomatic meetings held between the most powerful countries.
C) Diplomatic meetings involving heads of governments rather than lower level representatives.
D) High-level NGO meetings that run parallel to heads of government meetings.