Answer
They are collinear.
Work Step by Step
We see if the slopes between the points are the same to see if they are collinear:
$m = \frac{y_2-y_1}{x_2-x_1} = \frac{c-c+d}{b-a} =\frac{d}{b-a} $
And:
$m = \frac{y_2-y_1}{x_2-x_1} = \frac{c-c+d}{2b-a-b} =\frac{d}{b-a} $
The slopes are the same, so they are collinear.
