She has a friend who's much (pretty) than her.

The correct answer is 'prettier'. Here's why: When comparing two things, we often use the comparative form of an adjective. For short adjectives like 'pretty,' we usually add '-er' to make the comparative form. However, when the adjective ends in a 'y' that is preceded by a consonant, we change the 'y' to 'i' before adding '-er.' So, 'pretty' becomes 'prettier'. Correct answer: prettier