In this idiom question, choose the alternative which best expresses the meaning of the expression “build castles in the air”.

Introduction / Context:
“Build castles in the air” is a classic English idiom that appears in storybooks, essays and competitive exam papers. It uses a vivid image to describe a common human habit: thinking about grand but unrealistic plans. In this question you must choose the option that best captures the figurative meaning of the idiom, not its literal picture of actual castles floating in the air.

Given Data / Assumptions:

• The idiom given is “build castles in the air”.
• We must choose the closest meaning among the options.
• The options are: work tirelessly, fly, dream, film something.
• The focus is on vocabulary and idiom usage.

Concept / Approach:
The image of constructing castles in the air suggests creating beautiful but imaginary structures that cannot stand on real ground. The idiom therefore refers to making unrealistic plans, daydreaming or imagining grand achievements without practical basis. The correct alternative should reflect this idea of dreaming, especially in a fanciful or impractical way. We then compare each option with this core sense and eliminate any that talk about completely different activities like working hard or making films.

Step-by-Step Solution:
Step 1: Understand that the idiom is metaphorical, not literal; no real castles are being built in the sky. Step 2: Fix the meaning as daydreaming or making unrealistic, fanciful plans. Step 3: Evaluate option a) “work tirelessly”. This is about working hard, which is almost the opposite of idle daydreaming. Step 4: Evaluate option b) “fly”. This is a literal physical action and has no direct relation to making unrealistic plans. Step 5: Evaluate option c) “dream”. This aligns very well, especially in the sense of daydreaming about impossible things. Step 6: Evaluate option d) “film something”. This refers to shooting a movie and is unrelated to the idiom's meaning. Step 7: Conclude that option c) “dream” is the correct explanation.

Verification / Alternative check:
Consider a sentence: “Instead of working on his project, he keeps building castles in the air about becoming a billionaire overnight.” Here, the idiom clearly means he is lost in unrealistic dreams rather than taking practical action. Replacing it with “working tirelessly” would completely change the meaning, and “flying” or “filming something” would make no sense. Replacing it with “keeps dreaming about becoming a billionaire overnight” preserves the original sense, confirming that “dream” is the correct choice.

Why Other Options Are Wrong:
“Work tirelessly” is almost opposite to the idea of idle, unrealistic dreaming. “Fly” describes a physical activity and does not capture the notion of unrealistic planning. “Film something” is about making movies and is not connected with building imaginary castles in the mind. These distractor options are designed to see whether you recognise the idiom or only guess from isolated words.

Common Pitfalls:
Some candidates think that an idiom must have a very complicated explanation and may ignore a simple, correct option like “dream”. Others mix this expression with “castles” or “palaces” in real-estate contexts and imagine it has something to do with property construction. The best strategy is to learn a list of common idioms with short explanations and revise them regularly. Over time, the correct meanings will become intuitive, and such questions will feel easy marks in the exam.

Final Answer:
“Build castles in the air” means to dream, especially to make unrealistic or fanciful plans.