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The answer is D, The imperial powers believed that they were spreading a superior culture in Africa and Asia.
The reason why the imperial powers believed that they were spreading a superior culture in Africa and Asia can be found in the text in phrases like Send forth the best ye breed which implies that they should send the best "breed" of people (which they consider to be white people), to be send to Africa and Asia to do services for a group of prisoners. Additionally, as we can see in the first lines of this poem, the writer capitalizes the words white man throughout the poem, giving us a clear idea of how superior they thought white men were.
Therefore, the answer to this question is option D. The imperial powers believed that they were spreading a superior culture in Africa and Asia.