Final answer:
The word 'untie' is an antonym of 'knot', and the word 'straighten' is an antonym of 'wrinkle'.
Step-by-step explanation:
Untie is to knot as straighten is to wrinkle.
The word pair 'untie' and 'knot' are antonyms, meaning they have opposite meanings. Therefore, to find the word that is the antonym of 'straighten', we need to find a word that means the opposite of 'straighten'.
An antonym of 'straighten' is 'wrinkle', as when you straighten something, you remove wrinkles or make it smooth. So, the answer is 'wrinkle'.