Select the option that can be used as a one-word substitute for the given group of words.
Speech without prior preparation

* Extempore means spoken or done without preparation. Therefore, it is the perfect one-word substitute for "speech without prior preparation".
Let's look at the other options:

Debate: A formal discussion on a particular matter in a public meeting or legislative assembly, in which opposing arguments are put forward.

Lecture: An educational talk to an audience, especially to students in a university.

Rambling: Lengthy and confused or inconsequential speech or writing.