The correct answer is B. What caused Great Britain and France to declare war on Germany in 1939 was the invasion of Poland.
The German invasion of Poland was a military action of Nazi Germany aimed at annexing the Polish territory. The operation began on September 1, 1939 and the last units of the Polish army surrendered on October 6 of that same year. It was the trigger of the Second World War in Europe, as it forced the declaration of war by France and the United Kingdom, and ended with the Second Polish Republic.