The length of alimentary canal is more in herbivorous animals than in carnivorous animals because 

Cellulose is difficult to digest. Herbivores find it delicate to digest cellulose and have to depend upon bacteria in their gut for the digestion of cellulose. 
1. Biting and churning the food is also important in herbivores so that it becomes easier to destroy the cell wall. 
2. Therefore, herbivores have longer alimentary canal than carnivorous as the digestion is slower and they acquire smaller nutrients compared to meat. 
Hence, option C is the correct answer.