Answer :
The answer for this question is Y=1200+100x because the question is asking for you to find the total cost of the shipment of the packages. First of all, $1200 is already added to the shipment cost, so the first and last answer is incorrect. But the is, it can't be the third one because the truck is a one time payment, so you do not need to pay for it again.
Y = 1200 + 100x
Y equals the total amount it will cost
x equals the number of 50 lbs