The correct response is:
There will be less organic matter available.
This is due to the fact that as energy and organic matter move up the food chain (from plant to mouse to snake to hawk), there is typically a loss of energy at each trophic level (often around 90% loss), which means that less organic matter is available to the next level in the food chain. Thus, the amount of organic matter in the mouse would be less than that in the plant.