The case of arthur (described in your textbook), who suddenly experienced the delusion that he could save all the starving children in the world with a "secret plan," but whose symptoms lasted only a few days, was diagnosed with

Schizophrenia

Explanation: He most likely recieved the schizophrenia diagnosis. This clinical entity is a mental disorder that can cause delisions, hallucinations, sudden mood changes, etc.

A type of delusion is the grandiosity delusion where the person has an ilogical idea of having an ability that exceeds human capabilities, it is usually a supernatural ability.

depends on what they need