Answer: A. Felt that their efforts were not successful.
Explanation: In the given excerpt from "A Rumor of War" we can see the description of how the soldiers felt discouraged because they discovered that the men they had scorned as peasant guerrillas were a lethal enemy. From the given options, the one that best expresses the fellings of the soldiers about the discovery and the situation, is the corresponding to option A, they felt that their efforts were not successful, and that broke their confidence.
