Final answer:
The correct answer is 'more quietly'.
Step-by-step explanation:
The correct answer to complete the sentence is more quietly.
In comparative and superlative forms, adjectives take the form of 'more' + adjective for comparative and 'the most' + adjective for superlative. In this case, 'quietly' is the comparative form of 'quiet', so 'more quietly' is the correct choice.
Example: Evan was sleeping more quietly than everyone else in the room.
Learn more about Comparative adjectives