The Balkan region was referred to as the "Powder Keg of Europe" prior to World War I because: ethnic tensions created conflict.

Serbia was building a large army.

most of Europe's weapons factories were there.

Great Britain was competing with France for colonies there.

A. ethnic tensions created conflict.

Explanation:

in the balkan region, there were many different ethnic groups living there. and because there was tensions between the groups, it seemed that anything could set them off and "explode" that powder keg. unfortunately this is exactly what happened when archduke franz ferdnand wsa assassinated, thus world war i started.