Final answer:
The underlined word in the sentence is a preposition, which shows a relationship between words in the sentence.
Step-by-step explanation:
The underlined word in the sentence is preposition. A preposition is a word that shows a relationship between a noun (or pronoun) and other words in the sentence. In this case, the preposition 'on' is showing the relationship between the action of driving carefully and the road. Prepositions often indicate location, direction, time, or manner.
Learn more about Parts of Speech