Explanation:

- The given phrase is "an unlikely chance or occurrence, especially a piece of good luck."

- Option 1: Pauper – This refers to a very poor person who has no money to pay for their needs. It doesn't relate to chance or luck.

- Option 2: Messiah – This refers to a leader or savior, particularly in a religious context. It’s unrelated to chance or luck.

- ? Option 3: Fluke – This perfectly describes an unexpected piece of good luck or a chance occurrence. It's the one-word substitute for the given phrase.

- Option 4: Masquerade – This means a false show or pretense. It’s related to deception, not luck.