Question

which behavior would you need to change to make your public speaking skills more effective? a. varying your volume to add interest. b. speaking without any expression

Answer 1

A. Varying your volume

Answer 2

In order to improve your public speaking skills, to make them more effective, you would need to work on: A. Varying your volume to add interest.  That is the only reasonable improvement, because if you detract expressions from your presentation, you'll probably put the audience to sleep.  Your objective as the speaker is to keep the audience engaged with what you're saying.  Hope that helps.