Figure C having points (1,1), (b,1), (b,a) and (1,a) will be the most useful.
We are required to position a rectangle in the first quadrant in the most useful way possible.
As we can see that,
In figure A, the line joining (1,0) and (b,0) lies on the x-axis.
In figure B, the points (0,a) and (a,0) lies on the x-axis.
In figure D, the line joining (0,0) with (b,0) and (0,0) with (0,a) lies on the x-axis.
But, we need the figure to be placed inside the Ist quadrant.
So, all above options are discarded.
Hence, option C is correct.