Final Answer:
The blank line in the map key should be filled with "Communist Allies Occupied Zones." Option A is the answer.
Step-by-step explanation:
In the context of the map describing Europe in the spring of 1945, the information provided indicates the shading of countries based on their affiliations. Germany is shaded dark purple, representing the defeated Axis power. Purple-shaded countries like Italy, Denmark, Romania, and Bulgaria are also Axis countries. Green-shaded countries, including Great Britain, France, and Russia, are likely representing the victorious Allies. Given the historical context, the term "Communist Allies Occupied Zones" aligns with the presence of the Soviet Union in Eastern Europe during this period, as they occupied certain zones in countries such as Poland and East Germany.
"United Nations" does not accurately describe the situation in 1945, as the United Nations was established later in 1945 and wouldn't have been directly relevant to the occupation zones immediately after World War II.
Option A is the answer.