The author didn’t introduce the quote as an example of the quality of Mphahlele’s word choice or the beauty of his phrasing. So there’s no evidence that (A) is the purpose.

The author told us about Mphahlele’s goal when writing. But that doesn’t mean the author thinks this goal should be a goal in common for other novelists. We don’t know whether other novelists agree with Mphahlele or whether the author thinks they should agree with him. We’re simply getting a description of Mphahlele’s own view

This best captures the purpose of the quote. The rest of the paragraph tells us Mphahlele’s thoughts about what writing should do. The quote tells us more about that same topic.

Although Mphahlele mentions prose, poetry, and drama, that doesn’t mean he wrote in all three forms. And, even if he did, the purpose of the quote isn’t to tell us the forms in which Mphahlele wrote. These three forms are simply brought up to show that Mphahlele thought, no matter the kind of writing, it should provide social criticism.

Perhaps Mphahlele believes all three kinds of writing should deliver social criticism. But that doesn’t mean he sees no distinction among these genres. Maybe views them as distinct in that poetry follows certain rules about rhyming or number of syllables, and drama is designed to be performed on stage. In any case, even if he did think that there wasn’t a distinction among these forms of writing, that wasn’t the purpose of the quote. The quote was designed to show us Mphahlele’s thoughts on the goal of writing, not to show that he thinks prose, poetry, and drama are the same.