The correct option is B had better
Let’s examine the sentence: “You ____ watch what you are saying. The boss is very upset today.” There are two complete related sentences. By reading the second sentence we can understand that the action has already taken and boss is upset because of action. So, the first sentence would state the action that has happened. As the action has already happened, the tense of the sentence would be past. Hence, Option B would be the correct answer as that is the only option having verb of past tense.