Correct Answer:
C. 31 January 2020
The correct answer is 31 January 2020. The United Kingdom officially left the European Union on 31 January 2020 at 11:00 p.m. GMT (midnight in Brussels, 12:00 a.m. CET). This marked the formal withdrawal of the UK from the EU, ending 47 years of membership. The withdrawal came after the June 2016 referendum in which 51.9% of UK voters chose to leave. The period between 31 January 2020 and 31 December 2020 was a transition period during which the UK remained in the EU single market and customs union while negotiating the future relationship. The UK fully left the EU's single market and customs union on 31 December 2020, but the formal exit date was 31 January 2020. The options 30 January 2020 and 31 January 2021 are incorrect dates.