Explanation:
The phrase "dark horse" is an idiomatic expression that refers to someone or something that is initially overlooked or underestimated, but then performs unexpectedly well, often achieving success. In this context, the sub mariners are being described as the "dark horse" of the match, meaning they won the match in an unexpected or surprising way.

- A. played most brilliantly: While playing brilliantly could lead to a win, it doesn’t convey the surprise or unexpected nature implied by "dark horse."
- B. played below their usual form: This is the opposite of what "dark horse" means. It suggests underperformance, not unexpected victory.
- C. won unexpectedly: This correctly captures the meaning of "dark horse," as it indicates the sub mariners won in an unexpected or surprising manner.
- D. lost as expected: This contradicts the meaning of "dark horse," which implies an unexpected win, not a loss.
- E. won as expected: This is also incorrect because "dark horse" implies surprise, not something expected.

Therefore, C. won unexpectedly is the best choice.