Final answer:
The sentence that clearly contains an idiom is option (d), 'His mouth was writing checks his body couldn't cash,' which means someone is overpromising beyond their capabilities.
Step-by-step explanation:
An idiom is a phrase or an expression that has a figurative, or sometimes literal, meaning. An idiom’s figurative meaning is different from the literal meaning. When we look at the sentences provided, we need to identify which one contains an expression that doesn't make sense when taken literally and is commonly understood to have a different meaning within the culture. Option (d) 'His mouth was writing checks his body couldn't cash' is the sentence that most clearly contains an idiom. This expression means that someone is making promises or commitments that they are unable to fulfill or is boasting beyond their abilities. None of the other sentences (a, b, or c) contain an idiom; they are literal statements or direct warnings.
The American Civil War, enduring from 1861 to 1865, is a historic event often examined for its length and impact. Contrary to initial expectations on both sides for a quick resolution, the conflict dragged on for four long years. This underestimation of the war's duration demonstrates the unpredictable nature of war, reminiscent of how people overestimate their abilities in the saying about writing checks one cannot cash, albeit in a much different context. The idiom alludes to the human tendency to make assumptions or promises that reality may not support, similar to how both the Union and the Confederacy's leadership and people were unprepared for the prolonged struggle they faced during the Civil War.