The intergovernmental military alliance to which the united states, canada, and britain belong is called
a.
nafta
b.
the european union
c.
nato
d.
unasur

NATO. Stands for North Atlantic Treaty Organisation. The military alliance between them all. I actually learned this last week. 

A) incorrect, NAFTA is a North American free trade agreement which has nothing to do with the UK
B) incorrect, the US and Canada aren't in the EU
C) correct
D) incorrect, that's a Union of south american nations

TL;DR = C is the correct answer :)