The expression "he is laboring under the delusion of a serious nature" implies that the person is experiencing a strong belief or misconception that is significant or grave in nature.

Derilium is a disturbed state of mind or consciousness, especially an acute, transient condition associated with fever, intoxication, and certain other physical disorders, characterized by symptoms such as confusion, disorientation, agitation, and hallucinations.

Since this summarised the expression, the correct option is B.