Final answer:
Sebastian was treated with exposure therapy, which utilizes systematic desensitization and relaxation techniques to reduce his arachnophobia.
Step-by-step explanation:
The treatment used to treat Sebastian, who has a fear of spiders (arachnophobia), is best described as exposure therapy. This type of therapy involves a systematic desensitization process where Sebastian is gradually exposed to the feared object, in this case, computer-generated images of spiders, and the intensity and proximity of the exposure are gradually increased in a controlled way. The goal is to weaken the conditioned fear response by creating a new, less fearful association through repeated exposure while using relaxation techniques to manage anxiety.