Hey Ronald,
The stimulus tells us that in order for a general theory to achieve its purpose, it must explain every aesthetic feature found in any of the arts.
Gen art theory achieves purpose
explains features found in any art
Furthermore, it lets us know that none of the premodern general theories were able to achieve this purpose, because they failed to explain some aesthetic feature of music. Therefore the premodern gen theories failed to achieve the necessary condition, which means they failed the sufficient condition, and could not achieve their purpose.
Gen art theory achieves purpose
explains features found in any art (becuase the premodern theories couldn't explain all features of music)
Which means
Gen art theory achieves purpose explains features found in any art
And there is no way around this - unless we say well music actually isn't art. If that's the case, then maybe some of the premodern theories did explain every feature of art, meaning they also could achieve their purpose. Answer choice (D) states this.
Looking at answer choice (A) - we don't know that all general theories of art accomplish this. If the general theory of art
achieves its purpose , then sure, this would be true due to the logical relationship we already identified (Gen art theory achieves purpose
explains features found in any art). However, we don't know that all gen theories of art achieve their purpose - and in fact, this stimulus seems to be showing examples of some theories that certainly didn't achieve that purpose (i.e. the premodern general theories which failed to explain music).