Select the most appropriate meaning of the underlined idiom in the given sentence.

You must have a concrete project and not build castles in the air if you want to submit an application for a loan.

have unrealistic ideas

The most appropriate meaning of the underlined idiom in the given sentence is have unrealistic ideas. The idiom "build castles in the air" means to have unrealistic or impractical plans or hopes.

The other options are not as appropriate.

• Talk irresponsibly means to speak without thinking about the consequences of your words.
• Make unplanned buildings means to build something without a clear plan or purpose.
• Make unfaithful friends means to form friendships with people who are not trustworthy.