Final answer:
The detail that best contributes to the solemn tone of this excerpt is the instruction to stay quiet and be grateful. The correct answer is option C.
Step-by-step explanation:
The detail that best contributes to the solemn tone of this excerpt is "telling us to stay quiet / and be grateful." This detail suggests a sense of restriction and obligation, reinforcing the somber atmosphere of the passage. The mention of the president's family growing up in the mountains and the powerful spirits of their ancestors adds to the overall tone, but it is the directive to stay quiet and be grateful that has the strongest impact.