C. is the answer.
They are automatically paying the $128 to get in the door.
We don't know how many of each item they are getting, but we do know what each item costs. If they get 2 of anything, then the price would be multiplied by 2, etc.
So, multiplying the prices of the items by their unknown numbers x, y, z makes sense. But after those figures are discovered, they all have to be added together so that they know how much money they are spending in total.
If you were to plug in random numbers for the variables x, y, and z and solve the equation, the one written in C. would return an accurate value representing the family's total spend.