It is between c 12 and d 13
12 cartons will be 15
and 13 will be 16.3
4/3 because I typed in on my calculator
Another way to solve this is to use the Midpoint Formula. The midpoint of a segment joining points

and

is

So the midpoint of your segment is

Perhaps it helps to see that the x-coordinate of the midpoint is just the average of the x-coordinates of the points. Ditto for the y-coordinate of the midpoint; just average the y's.
I like A as the best one
420 = 100*price - (fixed costs)
220 = 60*price - (fixed costs)
If we subtract one from the other one, we get 200 = 40*price, price = 5
that said, fixed costs are 500 - 420 = 80
Now if we move -220 to the right in Answer A, we'll get:
y = 5x -300 + 220 = 5x - 80
So it looks like A describes the model best