**Answer:**

Figure C having points (1,1), (b,1), (b,a) and (1,a) will be the most useful.

**Step-by-step explanation:**

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.