Which of the following words is closest in meaning to “Enigma”?

Enigma means a puzzle or mystery.

• Correct answer - puzzle: it has the same meaning as Enigma.
• question: a query - wrong, it does not mean the same as Enigma.
• solution: an answer - wrong, it does not mean the same as Enigma.
• work: labour - wrong, it does not mean the same as Enigma.