Reasons why race mixing is good?

Educated guess: C.

Explanation:

Okay, so this is another example of needing the text for me to really give you the absolute, 100% proper answer. So if there was a way you could screen shot it and add it to your question, or send a link to the text, copy and paste it, etc. that would be extremely helpful, because I cannot read the mind of the writer just through the asked question, you know what I mean?

The question is asking "what was the author's main purpose in the text?", which means "what is the basic theme the author is trying to express?" and it sounds like it's subjective - meaning, from the author's POV, or opinion. Therefore, it's not necessarily a "factual" answer. It's about reading the text and finding out the author's opinion of altruism.

HOWEVER, I can make an educated guess based on the previous question you put up about the same topic (that I answered).

I think it's C. clearly, the author believes all the rest are true, but C. sounds most like a "main purpose" to me.