Final answer:
The most appropriate command form of the verb 'decir' (to say) is 'di'.
Step-by-step explanation:
To fill in the blank in the command form of the verb, we need to choose the appropriate verb form used to give a command or order. In this case, the correct command form of the verb 'decir' (to say) is 'di', so the correct answer is D. di.
Learn more about Command form of the verb in Spanish