The sentence primarily serves to illustrate David's feelings about his situation in the country, contrasting it with his previous experiences. Among the options provided, the best interpretation is:
C. The motionless horses "tethered to rusted poles" are a picture of how David now feels being in the country with "nothing better to do." The carousel is becoming symbolic of David's inner conflict.
This choice effectively connects the imagery of the carousel to David's emotions and inner conflict, highlighting his feelings of stagnation and lack of excitement in his new environment.