Final answer:
The underlined sentence builds tension as it signals a change and suggests a risky, pivotal situation that heightens the stakes and engages the reader.
Step-by-step explanation:
The underlined sentence from paragraph 8 builds tension because it interrupts a conversation with a sudden change, suggesting the potential appearance or occurrence of something significant. The reference to the need for evidence indicates a risky situation, as persons are plotting, yet unidentified, creating a sense of danger and urgency. This moment is crucial as it sets up for a potential confrontation, or a need to counterplot, with the plotting individuals, therefore escalating the stakes and keeping the reader engaged in anticipation of what might follow.