Final answer:
The excerpt from James Joyce's novel exemplifies modernism through its experimental narrative and focus on the inner experiences of the character, diverging from traditional storytelling.
Step-by-step explanation:
The aspect of the excerpt from A Portrait of the Artist as a Young Man by James Joyce that most clearly marks it as a work of modernism lies in its narrative style and the focus on the inner life of the protagonist rather than external events.
Modernist literature is characterized by experimental narrative techniques such as stream-of-consciousness, a focus on the psychological experiences of characters, and a break from traditional plot-driven storytelling. Joyce's novel is a prime example of modernist literature, as it illustrates a deep psychological portrait and challenges readers with its complexity, illustrating the movement's ethos to 'Make it new!' as coined by poet Ezra Pound.