The correct answer is C. must.
The word “must” is used to express a strong obligation or necessity. In this case, the speaker is saying that if the people want to succeed, they have no choice but to work very hard.
The word “will” is used to express a prediction or expectation. In this case, the speaker is not saying that the people will definitely work hard, but that they are likely to do so.
The word “ought” is used to express a recommendation or suggestion. In this case, the speaker is saying that the people should work hard, but they are not required to do so.
The word “should” is also used to express a recommendation or suggestion, but it is less forceful than “ought”. In this case, the speaker is saying that the people should work hard, but they are not obligated to do so.