I prefer jazz music because it is ...............than rock music.

The correct answer is "more relaxing". Because the sentence uses 'than', it indicates a comparison. When comparing two things using adjectives, we often use the comparative form of the adjective. For adjectives with one or two syllables, we usually add '-er' to the end (like 'taller' or 'faster'), or we use 'more' before the adjective if it's longer. Since 'relaxing' is a longer adjective, we use 'more relaxing' to show that jazz music is more relaxing than rock music. So, the completed sentence is: I prefer jazz music because it is more relaxing than rock music.