Final answer:
The word that best fills in the blank is 'nostalgic,' which indicates Harold felt a sentimental longing for his high school years when he heard the old song on the radio.
Step-by-step explanation:
When the old song came on the radio, Harold suddenly felt nostalgic for his high school years. The term nostalgia refers to a sentimental longing for the past, often for a period or place with happy personal associations. The excerpt you provided conveys a sense of profound melancholy related to a time when things were different, hence why the feeling of nostalgia is a suitable choice.
Being indifferent doesn't fit since it implies a lack of interest, which doesn't align with the sudden emotional response triggered by the song. Although the term melancholic can refer to a profound sadness about the irrecoverable past, it doesn't convey the same sense of fond, personal longing that 'nostalgic' does. Lastly, excited is not the best choice as it suggests a high energy or eagerness for what's happening rather than a wistful reminiscence.