The right answer to this question is option C. the traits acted upon natural selection were genes.
At the time Charles Darwin developed the evolution theory, there wasn't a single study about genes in all science scenery, that could be explained by the lack of technology to study them and even to know they existed in the first place.
So, his theory was basically talking about adaptation, if an organism can't survive a specific environment, he'll probably die until another generation comes in and this one can survive in this environment, passing their genes to the next ones. The lack of information was the knowledge about the genes we have today, that's why most scientists take only the new Darwin theory, that includes the study of genes.
