In this sentence, the correct choice is (D) because the first blank requires a verb, and "know" is the appropriate verb for this context. The second blank requires the noun "no," which fits the context of the sentence.
- First part: "You should know when to say" implies that one should have the knowledge of when to say something.
- Second part: "no" fits the context as it is the word being referred to in the sentence.
Thus, the correct answer is (D) know / no.