Final answer:
The prefix 'un-' changes the meaning of the word 'friend' to its opposite, resulting in 'unfriendly,' which means 'not friendly.'
Step-by-step explanation:
The prefix un- is commonly used in English to indicate the opposite of the original word. In the sentence: 'The new classmate walked into her new school, hoping that none of the other kids would be unfriendly,' the prefix un- is added to the word 'friend' to mean the opposite. Therefore, the correct answer is c Changes it to mean not friendly.