Final answer:
Yitzchak sings in the book 'Resistance' to keep up his morale, inspire others, and to defy the Nazis, exemplifying cultural and spiritual resistance during the Holocaust.
Step-by-step explanation:
In the book, Resistance, Yitzchak sings to keep up his morale, to inspire others, and to defy the Nazis. It exemplifies the broad nature of resistance during the Holocaust, which took on many forms, including cultural and spiritual acts aimed at undermining Nazi power and upholding the humanity and dignity of the Jewish people in extremely adverse conditions. Music, along with other cultural activities, served as a potent form of spiritual resistance, helping to maintain a sense of identity and community despite the efforts to dehumanize individuals facing oppression.