This is interesting. It's a split between C and D for this, as it covers both almost equally. However, I would go with C, seeing as it all falls under the umbrella of "cultural significance", even when talking about what purpose the drum is being changed for.
In this excerpt, the author discusses the cultural significance of the drum. He argues that the drum's original purpose was music-making and calling people to arms. However, after the people who used it underwent a cultural change, the purpose became one of displaying the north's political dominance over Central Africa and their allegiance to Islam. These changes were displayed by the new carvings that were in fact branding, and by the thinning that changed the sound of the drum.