The best answer to how the muscular system interacts with the digestive system is:
**C - The involuntary muscles of the digestive tract move the food through the system.**
The muscular system, specifically the smooth muscles in the walls of the digestive tract, plays a crucial role in peristalsis, which is the rhythmic contraction that moves food through the digestive system. While the other options describe functions related to the respiratory system (A), saliva production (B), and nutrient absorption (D) respectively, they do not specifically illustrate the interaction between the muscular and digestive systems as clearly as option C does.