What does the author mean by the phrase "build up"?

Ahistorian has discovered a letter written by an early roman emperor. she translates the letter and discovers that it contains orders to a general before a battle. she works to determine when the letter was written to figure out which battle the letter references.the historian in the passage is most clearly demonstrating which historical thinking skill? a. identifying causes and historical contextb. thinking chronologicallyc. making and evaluating arguments and interpretationsd. analyzing historical sources