Final answer:
The correct word to complete the sentence is 'recklessly'.
Step-by-step explanation:
The correct word to complete the sentence 'Harry drives _____' is recklessly.
'Recklessly' is the adverb that describes how Harry drives, indicating that he drives in a reckless manner. It modifies the verb 'drives' and answers the question of how he drives.
The word 'reckless' in option b is an adjective, not an adverb. Therefore, it is not the correct word to complete the sentence.