Correct Answer :
D. morals, theme
Nouveau riche refers to people who have recently gained wealth. The speaker is describing how these people are a constant theme for literature. Option (d) is right, because morals refers to the principles of what is wrong and right behaviour.
This fits well in the sentence, because then the speaker would mean: the manner (way or method in which something is done) and morals (principles of right and wrong behaviour) of the newly rich. Also, theme is a good fit for the second blank.