Select the most appropriate meaning of the given idiom.

Castle in the air

Unrealistic plan

The most appropriate meaning of the idiom "castle in the air" is an unrealistic plan.

The other options are incorrect.

• "Acquire something honestly" does not mean an unrealistic plan. Acquiring something honestly means to get it in a fair and legal way, while an unrealistic plan is a plan that is not likely to happen.
• "Very expensive" does not mean an unrealistic plan. Expensive means costing a lot of money, while an unrealistic plan is a plan that is not likely to happen.
• "Polluted air" does not mean an unrealistic plan. Polluted air is air that is contaminated with harmful substances, while an unrealistic plan is a plan that is not likely to happen.

The idiom "castle in the air" comes from the idea of a castle that is built in the sky. In the same way, a castle in the air is a plan that is not likely to happen because it is not based in reality. For example, you might say that a person who is dreaming of becoming a movie star has a castle in the air.