As no options were given to choose from, I would go ahead to explain the words and then suggest another word pair which is Exasperate : Calm
Explanation:
A bristle is thin and stiff strand of animal or human hair. The stiffness arises when the animal or person is responding in anger or fear to an external stimuli. This makes the hairs on the animal or person to stand erect and stiff
The word 'Relax' on the other hand connotes calmness and rest and is the opposite state of being bristle or angered.
Another word pair that best completes the analogy would be Exasperate : Calm
