B. It joins two independent clauses.
The subject is mother and the mood seems very demanding
To be really honest, I think its a matter of opinion on which sentence is the strongest conclusion. To me, I think choice C is the best answer because it paints the contrast between the dog's wild instincts versus loyalty. I guess the conclusion should also mention that despite the abuse, Buck still learned to trust a human (even though one could easily understand why the dog would never trust a human after the abuse), so it shows just how powerful loyalty can be.
