The third alternative is correct (C).
To make a speech, you should research a narrower topic, this will help you master the subject and give you confidence to argue with more knowledge. If you choose a very broad topic, you may be able to, but the chance of you getting confused is great. This can make you nervous about the topic.
In addition, it is very important that you organize all the sources of your research, quote them when you use them, and you should also take care of organizing your notes so that you can consult...