Correct Answer: Unrealistic plan

Solution : The correct choice is the second option.

The idiom " castle in the air" is used to describe a fanciful or impractical project or idea that is unlikely to be realised. It implies that the plan or dream is like building a castle in the sky, which is not grounded in reality and may be too ambitious or impractical to achieve.

Therefore, the correct meaning is, unrealistic plan.