if a diary is centered on the theme of self-affirmation, then the theme can be seen in conflicts developed by the diarist, or when the character has to make choices. Therefore, the correct answer is that a theme in diary is developed by the diarist by repeating the ideas in different ways.
I'm not sure what the lesson is, but the answer that makes sense the most is number. Number because most lessons in subject-verb agreement concerns the singularity and plurality of the subject at hand
