Final answer:
Stephen Crane's quote illustrates naturalistic themes by underscoring humanity's trivial presence and inability to influence the indifferent universe, aligning with naturalist literature that portrays humans as subject to external forces.
Step-by-step explanation:
The irony in Stephen Crane's statement "A man said to the universe: 'Sir, I exist!' 'However,' replied the universe, 'the fact has not created in me a sense of obligation.'" reveals naturalist themes by highlighting humanity's insignificance in the grand scheme of nature.
This notion fits best with option D) It shows humanity's helplessness against external forces. In naturalistic literature, individuals are often depicted as being at the mercy of the universe and external realities that are beyond their control, a theme that Crane emphasizes through this conversational interchange between a man and the indifferent universe.