Concept:The idiom “work like a dog” means to put in great, continuous effort.
Explanation:The phrase describes working extremely hard, often with little rest.
It does not mean working for a reward, as that refers to motivation.
It does not mean simply working faithfully, as that refers to loyalty.
It also does not mean making a mess while working.
Instead, the phrase directly expresses doing work with intense and strenuous effort.
Answer:Therefore, the correct meaning is: “To work very hard.”
So, option C is the correct answer.