Final answer:
Siddhartha seeks to learn the nature of Nirvana from the river, which symbolizes the flow of life and the interconnectedness of all things in Hermann Hesse's novel.
Step-by-step explanation:
In Hermann Hesse's novel Siddhartha, the protagonist Siddhartha seeks spiritual enlightenment and hopes to learn a profound spiritual truth from the river. Specifically, he looks for the nature of existence and hopes to understand the ultimate truth of enlightenment, akin to the concept of nirvana in Buddhism. The river, with its constant flow and ever-changing nature, represents the eternal flow of life and the interconnectedness of all things. In observing his reflection in the river, Siddhartha contemplates the cyclical nature of life and the unity of creation, leading him to the realization of time's illusion and the deeper truths of existence.
Buddhism, which is a central theme in the novel, defines nirvana as the state of ultimate peace found in the extinction of all desire, as indicated by the student's multiple-choice question. It is this very understanding that Siddhartha seeks from his contemplations on the river, thus the correct answer is 'a. the nature of Nirvana'.