The phrase, ''they were unable to engage in combat due to racial prejudice'', is the one that describes the role of most African-American military personnel during the World War I. Therefore, the option A holds true.
During the times of the First World War, the American armies focused on creation of a bigger army, and as a result, ordinary citizens were being deployed into the army. The African-American soldiers, however, were unable to represent the militaries in the First World War, because they had racial prejudices against them.
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