Based on the available information, the correct option is D. A square with sides parallel to the coordinate axes.
To determine the representation of the equation x - y = 3 under the usual 2-dimensional coordinate system, rearrange the equation to the standard form of a linear equation, which is y = mx + b, where m is the slope and b is the y-intercept.
Rearranging the given equation, we have:
x - y = 3
y = x - 3
From this equation, observe that the slope is 1 (the coefficient of x) and the y-intercept is -3 (the constant term).
Now, let's analyze the options:
a. A square with side units: This option is not applicable since the equation does not represent a square.
b. A parallelogram which is not a rhombus: This option is not applicable either since the equation represents a straight line, not a parallelogram.
c. A square whose sides are not parallel to the coordinate axes: This option is also not applicable as the equation represents a straight line, not a square.
d. A square with sides parallel to the coordinate axes: This option is correct.
The equation y = x - 3 represents a straight line with a slope of 1, passing through the point (0, -3). The line is parallel to the x-axis and y-axis, forming a square with sides parallel to the coordinate axes.
Therefore, the correct option is D. A square with sides parallel to the coordinate axes.