Final answer:
The theme that opens the third movement of Beethoven's Symphony No. 5 is known as the 'rocket theme' or 'motto' theme. It consists of four short, powerful notes followed by a longer note, creating a rhythmic motif that is repeated throughout the movement.
Step-by-step explanation:
The theme that opens the third movement of Beethoven's Symphony No. 5 is known as the 'rocket theme' or 'motto' theme. This theme consists of four short, powerful notes followed by a longer note, creating a rhythmic motif that is repeated throughout the movement. It is characterized by its energetic and driving quality, creating a sense of urgency and excitement in the music.