A paradox in literature is when a sentence or phrase contradicts itself but is still somehow accurate or could be if investigated further.
I see that the option “Her sister is awfully pretty” might seem to be the answer, but it is not. In colloquial/informal English the word “awfully” essentially means “very.”
The answer you seek is, “You have to be cruel to be kind.” This is the answer because it is a phrase that conveys a meaning that contradicts itself but could possibly be truthful or accurate in some shape or form.
I hope this helps, and apologizes if I am wrong; please ask if you have any questions.