Answer:
d) Change slides not less than every 30 seconds
Step-by-step explanation:
a) Use several different fonts and font sizes throughout the presentation
Having different fonts and font sizes in your presentations will most likely distract your audience. Therefore, it is recommended to use same font and size throughout your presentation.
b) Present as much written material as possible on each slide
This is clearly not a good idea to put too much verbose in your presentation. The audience is less likely to focus on you if you put too much text in your slides. One should always keep the key points in the slides and explain them verbally to your audience.
c) Vary the slide background from slide to slide
This is similar to option A and might distract your audience, therefore not recommended.
d) Change slides not less than every 30 seconds
If you are changing the slides very fast then your audience might have trouble understanding or absorbing the content of your slides. Therefore, it is recommended that you should not change slides less than 30 seconds.