You tell her about that secret. She can tell your father A. Should
B. Shouldn't
C. Must
D. Mustn't

D. mustn't

Both "must" and "should" are model verbs. The term "must" is commonly used to express any unavoidable requirement or obligation. On the other hand, "should" is used as a probability, obligation, advice, recommendation, conditional, and exceptional mood.

since the sentence contains an unavoidable requirement that the secret must not be disclosed to her because she can tell your father, must is used.

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