Answer:
Speech
Step-by-step explanation:
The use of the words "straigh-forward" gives us a clue, we need to express a message that is simple and easy to understand. That means there is no space for ambiguity, the words used need to be clear. They need to transmit what the author wants to say, no more, no less. The best way to achieve this is by a speech. The speech allows the author to deliver a spoken message to an audience face to face. There, by using his speaking skills, he can say exactly what he thinks in the way he wants. He can convince the audience of something, inform it, motivate it or just entertain it.
The other options leave a lot of room for misinterpretation as the real meaning, although developed by the author, needs to be discovered by the audience.