The best response is:
Speech and gestures combine to suggest a general impression.
This answer is correct because the passage describes how silhouettes spread out to represent the prairie, and the primary emphasis is on how Gretchen Schriver's speech is combined with the visual elements (the silhouettes) to create an overall impression of her family's journey and the landscape. The imagery of the prairie grass "waving mile after mile" suggests a broader understanding rather than a precise, detailed representation, which aligns with the idea of a general impression.