Final answer:
The topic sentence in the given set of sentences is (5) "What we call the novel is thus an immensely varied literary form," as it introduces the broad theme of the variety within the novel genre.
Step-by-step explanation:
The topic sentence is usually the one that introduces the main idea around which the rest of the text revolves. In the given set of sentences, the one that stands out as the topic sentence is (5) "What we call the novel is thus an immensely varied literary form." This sentence presents a broad statement about novels as a diverse literary genre, setting the stage for the subsequent sentences to provide examples in relation to this broad theme. The other sentences discuss specific authors and their works, thereby supporting the notion presented in the topic sentence about the variety found within novels.