Answer:
Granny's phrase 'The laugh's on him' is an idiomatic expression indicating that the person who initially laughed is actually the target of the joke or has made a mistake.
Step-by-step explanation:
When Granny says, "The laugh's on him," she means that the person who thought they had reason to laugh is actually the one being laughed at because they are mistaken. This expression indicates that the supposed joke has backfired, and the humor is now at the expense of the person who originally laughed. Therefore, the correct answer to the question is B) He is mistaken.