Introduction:
In the context of the NCERT Solution Chapter 1 - The Last Lesson from the Flamingo textbook, the phrase "the key to their prison" refers to the power of education and knowledge to liberate individuals from their limitations and narrow perspectives.

The Last Lesson:
"The Last Lesson" is a short story written by Alphonse Daudet, which revolves around the theme of the importance of education and the impact of language on cultural identity. The story is set in a small village in Alsace, which was recently annexed by Germany. As a result, the German language was imposed as the medium of instruction in schools, replacing French.

Understanding "the key to their prison":
1. Symbolic Representation: The phrase "the key to their prison" symbolizes the transformative power of education. Just as a key can unlock a prison cell and set a person free, education can liberate individuals from ignorance and narrow-mindedness.

2. Language as a Barrier: In the story, the protagonist, Franz, is deeply attached to his French language and culture. The imposition of the German language in school becomes a metaphorical "prison" for him and his classmates. They are trapped in a situation where they are unable to express themselves or connect with their own identity.

3. Loss of Cultural Identity: By taking away their native language, the German authorities aim to suppress the French cultural identity of the students. Franz and his classmates feel a sense of confinement, as they are cut off from their heritage and traditions.

4. The Power of Education: However, the phrase "the key to their prison" suggests that education can act as a liberating force. Even in the face of adversity, Franz's teacher, Mr. Hamel, demonstrates the importance of knowledge and learning. He emphasizes that language and education are not just academic subjects, but the essence of one's cultural identity and freedom.

5. Regret and Realization: Towards the end of the story, Franz regrets not having valued his French lessons before. The phrase "the key to their prison" represents the moment of realization for Franz and his classmates, as they understand the true significance of education and their cultural heritage.

Conclusion:
In the context of the NCERT Solution Chapter 1 - The Last Lesson, the phrase "the key to their prison" symbolizes the transformative power of education and knowledge. It represents the liberation of individuals from ignorance, language barriers, and the suppression of cultural identity. Through this story, Alphonse Daudet highlights the importance of valuing education and one's heritage, even in challenging circumstances.