To find the word that best fits the feeling described in the sentence, we need to analyze each option in relation to the emotion evoked by the smell of old books, which is linked to childhood memories.
The sentence mentions that the smell of old books "often fills me with a sense of reminding me of my childhood days." This suggests a longing or wistfulness for the past associated with a positive and affectionate memory.
Therefore, the word that best fits is nostalgia, as it precisely describes the emotion of remembering and longing for the past, particularly one's childhood.