The best answer is letter "B. the topic, medium, and supporting sentences".
Explanation:
The main idea of a text is usually expressed with a clear statement in which is possible to identify the topic of conversation and/or interview/lecture/discourse. The medium is also important because it provides the kind of ideas expressed, whether they are formal or not, for example. Moreover, the supporting sentences are responsible for making the main statement a true one, as their function is to provide evidence and more context to the main idea.
