The answer would be d. This is because parallel lines have the same slope. Lets say the first line is f(x). The slope of this would be undefined because all the x-coordinates are the same, so if you tried to find the slope, you would use the slope formula, y^2-y^1/x^2-x^1 and if we used the points (1, 1) and (1,2) we would end up with 2-1/1-1 = 1/0. Because there is a zero in the denominator, it makes f(x) undefined. And as I said before, parallel lines have the same slope, f(x)' would also be undefined.