Answer:
All of the above logically could be applied in this context; however, if you are to choose one, I would think your best bet is "Vary Tone and Emphasis."
Explanation:
There's a lot that goes into making a speech more enthralling for the audience, especially depending on what kind of presentation you're giving. Generally speaking, I believe the tone and emphasis are most important for all speeches, so that probably sounds like your best bet.
