The correct answer is C. The essay has been using numerical connectors to add each additional paragraph. Thus, the linker "third" in this option is more appropiate than the others, since it clearly guides the reader. Remember that connectors are intended to make clear the logical connections between the ideas, especially between paragraphs. Thus, if sequencial connectors were adopted from the start, it is better to keep that sequence. "Finally" is possible but it is better used for conclusions.
