There are several different answers that can complete the sentence. One such approach would be that "Based on the description of the lieutenant dividing up the coffee, you can best make the inference that he had little idea of what was to come." Crane's description reveals that the Lieutenant's focus was on the rationing of the coffee and little else. It did not occur to him that the object of his focus was taking place in the midst of a battlefield with the realities of war not very far off:
The lieutenant was frowning and serious at this task of division. His lips pursed as he drew with
his sword various crevices in the heap until brown squares of coffee, astoundingly equal in size,
appeared on the blanket. He was on the verge of a great triumph in mathematics...
Through such a description,we can infer that the lieutenant's focus was placed squarely on the division of coffee. This mundane task occupied much in way of importance, almost overshadowing the condition of war that proved to be inescapable.
Another approach that can taken to the sentence would be that "Based on the description of the lieutenant dividing up the coffee, you can best make the inference that he was completely surprised by his injury. The focus that he was giving toward dividing up the coffee prevented the lieutenant from really taking the necessary precautions and steps to keep himself safe. It is for this reason that "The sound of his hoarse breathing was plainly audible." The lieutenant's focus on dividing up the coffee almost precludes any real sense of understanding about him being hit and prevents the lieutenant from engaging in any sort of reflection about its implications. It is in this light where one can see that he was caught completely surprised by his injury, an awkwardness that does not leave the lieutenant throughout the narrative in the relationship between himself and his injury.