Why does Dysart see Alan as a trigger for his professional doubt?

Explanation:

Dysart sees Alan as a trigger for his professional doubt because Alan challenges his expertise. As a psychiatrist, Dysart is used to understanding and helping his patients. However, Alan's case is different. Alan's passion for horses and his religious belief in the god Equus make Dysart question his own understanding of the human mind and what it means to be 'normal'.

Therefore, the correct answer is : (B)