Final answer:
In music, adding in unstressed syllables results in shorter notes that move quickly to the next stress, which causes the rhythm to become more rapid.
Step-by-step explanation:
In music, adding unstressed syllables 'crowds' in between the strong stresses, demanding shorter notes that move quickly. These are often called passing notes, which lead quickly and smoothly from one important note (or stress) to the next. So the correct answer is C) Quickly. As these shorter, unstressed notes or syllables are added between the stronger, longer stressed notes, the resulting rhythm tends to move at a more rapid pace.