The key factor in Britain's vote to leave the European Union was:

A) Control over Britain's monetary policies
B) Independent authority to negotiate external trade deals
C) fear over the effects of mass migration
D) the right to develop its own labor laws

Fear over the effects of mass migration was a major factor in the Brexit vote. Many Britons were concerned about the impact of continued migration from other EU countries on social services and job opportunities. This sentiment was exploited by the Leave campaign. While other factors such as control over monetary policies and independent authority to negotiate external trade deals were also important for some voters, the fear of mass migration was the key factor that drove many to vote for Brexit.