Explanation:

- A pipe dream

- This idiom means a hopeful but impractical and unlikely plan. It's a wish or idea that's almost impossible to achieve in reality.

- Option 1: An illusory plan

- This option matches the definition of a pipe dream. It refers to an unrealistic or impractical plan, something that might sound good but is unlikely to happen.

Correct Answer

- Option 2: A secret

- This option does not align with the meaning of a pipe dream. A secret involves confidential information that's hidden from others.

- Option 3: A realistic dream

- This is the opposite of a pipe dream. A realistic dream is something attainable or achievable, which is not what a pipe dream is.

- Option 4: A clever strategy

- While clever, a strategy is purposeful and usually practical, unlike a pipe dream.

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