The correct answer is A.
Before giving a speech, the speaker needs to know very well the audience he is going to be addressing. This is a key point because he needs to adapt his language and vocabulary to the people who will be listening to him.
He also needs to adapt to the time he will be given to speak, so as not to make it too short or too long. He will have to practice the speech and time himself, to be sure how much time he will need.
Finally, the speaker needs to be completely proficient on the topic he has chosen. He has to have concise and exact data to provide to his listeners. And he needs to investigate further, to be able to answer any questions the audience may have.
However, even thought the amount of research done on the subject is quite important, it would be the less likely part to be taken into account.
