prisoners-of-war|

The Nazi kept those ________ in their concentration camps.

 A. prisoner-of-wars

 B. prisoner-of-war

 C. prisoners-of-wars

 D. prisoners-of-war

Analytical title

Number of test compound nouns. The sentence means: Nazi prisoners of war in a concentration camp. prisoner-of-war time varying its plural form, which is simply subject to plural nouns prisoner become, so choose D