The cost of transport of tons is the point of intersection between the line and the X axis
Now we see that the point O tons, corresponds in the line to the point Y=1600
this the answer a)
For answer b) the point 1 corresponds to 2000
for answer c) the cost increase per ton is 400 , that is because 2000-1600= 400, and the line is inclined with a slope equal to 1