In simple terms, scrubbing the stain only made it worse. Exacerbating means to make something worse than what it already is.
The answer is the last choice, or D.
When you summarize, you're supposed to gather the most important points and rewrite them in a shorter and more simplified fashion. The first three choices include mostly everything you should do when writing a summary. Usually when you summarize, it's for a concluding paragraph, so writing an introductory paragraph wouldn't be it.
