In this question, it is given that :
The total amount of money M earned by washing cars at the school fundraiser is proportional to the number of cars , c , washed at the constant Price ,p .
That is , for number of cars,1, we will get

For number of cars,2, we will get

For number of cars,3, we will get

Therefore for number of cars,c , we will get

Correct option is D .