Final answer:
Assonance is demonstrated in the phrase 'astounding and off-putting' by the repetition of the initial 'o' vowel sound.
Step-by-step explanation:
The subject here involves the use of assonance, which is the repetition of vowel sounds in close proximity within a text to create a pattern or a rhythm. When looking for an example of assonance, we need to identify options where initial vowel sounds are repeated closely together. In the choices provided, astounding and off-putting would provide the best example of assonance because both words start with the same vowel sound 'o', creating that pattern of similar sounds close together.