In order to answer this, you must have a clear understanding of the difference between a primary source and a secondary source. A primary source is one that has been written in the time period being studied. A secondary source analyzes key points, often based off of primary sources.
Knowing these definitions, we can conclude that the correct answer is (c) an academic analysis of key battlefield strategies. An academic analysis will provide insight on the topic being studied (in this case battlefield strategies), however it is typically not written with a direct connection to the time.
