Final answer:
The mood of the excerpt is one of melancholy, characterized by the unexpected death of a very young individual and the surrounding circumstances of loss and war-related suffering.
Step-by-step explanation:
The mood of the given excerpt implies a sense of loss and unexpected tragedy. The use of phrases such as 'very young' and 'had died of pneumonia' in combination with the suddenness of the event ('had been sick only a few days') convey not just the finality of death but its surprising nature and its impact on those left behind. The mention of marriage only after the major was 'definitely invalided out of the war' adds a layer of empathy for both the major, whose life has been significantly affected by war, and his young wife, whose life ended prematurely. The predominant mood here is melancholy, amplified by the sense that this is a narrative about somebody who has undergone significant suffering only to endure further loss.