The correct answer is D) by hoisting the code pennant, and keeping it flying during the entire communication. This is the correct answer because according to maritime protocol, a warship will typically hoist the code pennant to indicate that it wishes to communicate with a merchant ship. The code pennant is a distinctive flag that signifies the warship's desire to establish communication, and it is kept flying for the duration of the exchange to maintain the communication link. The other options are incorrect because they do not accurately reflect the standard procedure used by warships to initiate communication with merchant vessels. Hoisting the merchant ship's national ensign or identity signal, or the flags "MV," would not be the appropriate way for a warship to indicate its intent to communicate.