Final answer:
Neville's boggart takes the form of his Professor, Professor Snape.
Step-by-step explanation:
In the "Harry Potter" book series written by J.K. Rowling, a boggart is a magical creature that takes on the form of the viewer's worst fear. When someone encounters a boggart, it appears as whatever the individual finds most frightening. The spell used to face a boggart is "Riddikulus," which, when cast with the right incantation and intent, can transform the creature into something humorous, thereby neutralizing its fear-inducing effect. In the Harry Potter series, Neville's boggart takes the form of his Professor, Professor Snape.