When people are forced to face situations that make them extremely fearful, their fear often begins to extinguish. this fact underlies the use of a procedure known as: ?

I believe the answer is: systematic desensitization

systematic desensitization refers to a situation when people keep gradually make lesser response as we got used to the stimuli
For example, an experienced fire fighter would much more likely to have enough courage to burning building compared to normal civilian do to their past experience.