Final answer:
The correct adverb is 'most speedily.'
Step-by-step explanation:
The correct adverb in the given sentence is most speedily. The adverb 'most speedily' is used to describe how the police car arrived. It represents the superlative degree of the adverb 'speedily.'
'Speedily' is already an adverb, but when comparing two or more things, we use the superlative degree to show the highest degree of the quality being described. Therefore, 'most speedily' is the correct adverb in this sentence.
Example: The turtle moved most slowly compared to the other animals in the race.
Learn more about Adverbs