What was the one thing that pulled together or characterized classic China?
It may be that your textbook has a particular answer that you are supposed to provide. This is the sort of question that could be answered in many ways and our interpretation may not be the same as the one in your book.
That said, I would argue that the one thing that pulled classic China (by which I mean the Eastern Chou period) was Confucianism. During this period, China was recovering from a time of great social and political upheaval. People appeared to be searching for new ways and new values that could guide them. In this climate, Confucianism arose as a set of values for ordering human society. The classic texts of Confucianism came to be the source from which Chinese thinkers would try to find answers for their problems.
Confucianism, then, was the one thing that was common to Chinese people during this period. It was the one thing that was used to hold together a vast population in one state.