Overall, mood in literature is a literary element that evokes certain feelings or vibes in readers through words and descriptions. Usually, mood is referred to as the atmosphere of a literary piece, as it creates an emotional setting that surrounds the readers. Mood is developed in a literary piece through various methods - setting, theme, tone, and diction.
According to the question above, the correct answer is letter C: “The nervous mood helps the reader understand how Wills's forgetfulness has led to a genuinely frightening experience for him”.