The most credible and relevant source for this essay would be a series of firsthand accounts of soldiers reminiscing about their duty shifts, as shown in option D.
The report of soldiers who participated in the war in Iraq is a document that shows what soldiers lived in that war. This is a real document with true and reliable information, as the soldiers are presenting their personal experiences.
In this case, these accounts are an efficient source for the essay on the lives of soldiers.
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