Which of the following is the main disadvantage of extemporaneous speaking?
the speaker cannot change direction based on the audience's response.
the speaker will often not know enough about the topic.
the speech can often sound insincere and rehearsed.
the speaker may forget ideas or make grammatical mistakes.
all of the above

I believe the correct answer is that the speaker may forget ideas or make grammatical mistakes.
Extemporaneous speaking has a lot to do with improvisation - it is a speech where you get a topic you haven't prepared for, and then you have 10-20 minutes to prepare and speak. This way, you cannot memorize everything that you wanted to say, and given that you aren't completely ready to speak, you may make some mistakes.

The correct answer is