O C. People must use their mistakes to build a better future.
Explanation:
A theme of a speech or a text is the main idea of the given text that the author wants to relay to the readers/ audience. In other words, we can say the theme of a speech is the main purpose of the speaker.
Among the given sentences, sentence C best represents a speech's theme. This is because the statement that <em>"people must use their mistakes to build a better future"</em> revolves around the overall idea of learning from past mistakes and getting better through the lessons learned.
