Answer:
D. Contrast how each author’s message about the theme is different.
Step-by-step explanation:
The means to compare any two or more differing things is best approachable by means of taking the contrasts of the things or ideas that is/ are being compared. So, in the case of two authors' use of themes in their separate works, it is best to find the contrast in their message about the theme, and see how different they are.
By using just the theme used by one author is not sufficient to fully compare the themes of the two authors. Nor a specific cultural theme is satisfactory, for there might be cases where there are no cultural themes available in the stories/ plot. And if the themes of the books that are directly stated are only taken into consideration, then there can also be some discrepancies in the final comparison of themes. Lastly, the very word "to compare" asked in the question directly relates or refers to "contrasting" of the themes. To compare means to take two or more things into consideration and see/ find their differences. To contrast also means the same. Thus, the best way to compare the ways that two authors use theme in their works is to contrast how each author's message about the theme is different.