C. Is it true that small, average, and great people talk about people, things, and ideas respectively?
Option C is the best way to combine the sentences because it presents the original statement and accurately captures the structure and meaning of the original sentences. It begins with the question "Is it true," which reflects the inquiry about the validity of the statement. It then proceeds to list the categories of people and what they talk about, using the words "small, average, and great" to describe them and "people, things, and ideas" to specify the subjects of their conversations.