Final answer:
Option C most clearly marks the shift in tone in the poem by using intense language to evoke strong emotions, contrasting with previous tones.
Step-by-step explanation:
In poetry, a shift in tone is often marked by a change in language or imagery that signals a move to a different emotional state or perspective. In the provided options regarding the lines that best mark the tone shift from the first to the second tone, option C (This is the hymn of the mud—the obscene, the filthy, the putrid, / The vast liquid grave of our armies) most clearly represents a tone shift. This shift is signaled through the use of intense, vivid language that evokes strong emotional responses and conveys a sense of gravity and repulsion, forming a contrast with any previous lighter tones or more neutral descriptions.