A, All about hip-hop music.
Choice A is a more broad research topic because just tells about hip-hop in general, rather than focusing on a certain aspect of hip-hop.
Choices B, C, D, on the other hand, focus more on how hip-hop is “[a] voice of outrage,” hip-hop’s rhyme and meter, and how hip-hip originated from block parties. They focus on specific things related to hip-hop.
