Answer
$B: (a,0) \\ C: (a,a) \\ D: (0,a)\\ A: (0,0)$
Work Step by Step
We call the point A at the origin, which is $(0,0)$. Saying that there is a side length of x, we find the following coordinates:
$B: (a,0) \\ C: (a,a) \\ D: (0,a)\\ A: (0,0)$