D is correct answer because it is derived from many different styles and genres in the 1960s and incorporated many different influences, which led to being a music of protest among the other things. Artist like James Brown made this messages even powerful.
A is wrong as classical music is not a popular type of music where this themes would be introduced.
B is also wrong as folk music usually describes the life of individuals and has easier notes.
C is also wrong as jazz is more focused on improvisation than on expression through lyrics.
