The correct answer is D. (extinction). Extinction refers to the decrease of the conditioned response when there is no unconditioned stimulus presented after the conditioned stimulus. In this example, the unconditioned stimulus is fire, which leads to, for example, running outside the school (unconditioned response). The fire alarm sound (conditioned stimulus) was associated with fire and also lead to the same response (conditioned response). When the alarm goes off for a lot of time without fire, the response decreases and that is why Susannah and Tia did not even flinch.