Answer: The word that best describes "building up" an idea in your paper is developing.
What is Developing?
Developing means expanding or elaborating on an idea by providing additional information, supporting evidence, and examples. When you develop an idea in your paper, you are making it more clear, detailed, and convincing to your readers.
What is Refining?
Refining means improving the quality or accuracy of something by making small adjustments or corrections. While refining an idea can be part of the process of developing it, refining does not convey the sense of expansion or elaboration that building up an idea implies.
Therefore, the best option is B. Developing.