Final answer:
Thematic development in music involves manipulating and transforming a musical idea or theme throughout a composition in various ways. This can include treating a motive in sequence, breaking up a theme into motives, or expanding a motive into a long melody. The correct answer is option B.
Step-by-step explanation:
A. Treating a motive in sequence, C. Breaking up a theme into motives, and D. Expanding a motive into a long melody all represent different types of thematic development. Thematic development refers to the ways in which a musical idea or theme is manipulated and transformed throughout a composition. For example, treating a motive in sequence means presenting the motive in a sequential pattern, repeating it at different pitch levels. Breaking up a theme into motives involves dividing the theme into smaller melodic fragments or motives, and expanding a motive into a long melody means elongating and developing a short musical idea into a more extended melody.