Final answer:
The last part of the development in movement 1 of 'Eine kleine Nachtmusik' is marked by a 'Return to the main theme,' providing closure to the form before the recapitulation. The correct option is D.
Step-by-step explanation:
Describing the last part of the development in movement 1 of Eine kleine Nachtmusik, option D) Return to the main theme is the correct answer. In classical sonata form, which Eine kleine Nachtmusik adheres to, the development section leads into a recapitulation where the main themes presented in the exposition are revisited.
This moment in the music is characterized by a restatement of the opening material, often returning in the original key, providing closure and balance to the form.
Rapid harmonic shifts, crescendos in dynamics, and sudden tempo changes may occur throughout the movement, but the defining feature that marks the end of the development and the transition into the recapitulation is indeed the return of the main theme. The correct option is D.