The most effective way to provide elaboration in a paragraph is to a. cite passages from the text b. revise the introduction c. underline important parts d. include names of all of the characters

Most likely A. 
B, won't matter much. It will draw readers in, but it won't change the real information.
C. Underlining important parts is sort of useless, because you are only adding emphasis on some parts and not really providing much more information.
D. Honestly, why are you even elaborating if you don't even have the character names yet.

China plays a major role in population and china is also a well developed country