Answer:
A. Germany
Explanation:
After the Second world war, the allies all split up Germany between them; 4 splits total - France, UK, USA, USSR.
The allies also split Berlin between themselves, with the west side going to the western allies and the east going to the Soviets.
The USSR's split of Germany (East Germany and East Berlin) was logically made communist by the Soviets, and this was seen as a threat by the USA due the communist influence being essentially right on their split of Germany's border (Both in West Germany and West Berlin). Not to mention the repeated attempts to get more of Germany (Especially Berlin) by the Soviets.
Therefore Germany was the first challenge.
