Concept:The sentence contains an idiomatic expression that means planning impossible or unrealistic things.
Explanation:The phrase "builds castles in the air" refers to a person who makes dreams or plans that cannot be achieved in real life.
These ideas have no practical foundation or real chance of succeeding.
The words are not meant literally, so the person is not an actual builder of castles.
The expression also has nothing to do with earning or accumulating money.
Instead, it shows that the person is occupied with thoughts of things that are impractical and impossible to accomplish.
Hence, out of the four given interpretations, the most suitable one is that the person thinks of impracticable things.
Answer:B. thinks of impracticable things