You are writing a letter to the editor of your city’s newspaper about a local law that you want changed. what should you do in the body of the letter?

introduce your main idea

develop your argument

sum up your arguments

capture readers with a strong lead

While the first choice would be something you MUST do in such a letter, it is NOT what you do in the BODY of the letter. You must do that in the introductory paragraph.
The third choice is also something you must do in the letter, but not in the body, either. The third choice would be written about in the conclusion.
The fourth choice is important as well, but not what you really do in the body paragraph. It's something that's more likely to be in the introduction than the body.
So, that leaves the second choice, B. develop your argument. This would be the correct answer for your question. 
In a body paragraph, you write about and develop on your topics mentioned in the introduction.