Final answer:
An insistent quality in theme 2b of movement 1 in Eine kleine Nachtmusik is achieved through the use of repetitive dynamics.
Step-by-step explanation:
The insistent quality in theme 2b of movement 1 in Eine kleine Nachtmusik is created through the use of repetitive dynamics. This technique involves repeating the same dynamic level (loud or soft) over and over again to create a feeling of insistence or urgency. It's akin to someone knocking on a door repeatedly with the same intensity, demanding attention. In music, such repetitive dynamics can highlight a particular theme or motive and make it stand out, conveying its importance or emotional weight to the listener.
While the other options like rapid melodic intervals, sudden key changes, and syncopated rhythms can also contribute to the creation of an 'insistent' feeling in music, they are not the primary techniques used in the specific part of the composition in question.