To find the area of the least-sized square that can be inscribed in a larger square (with its vertices touching the sides of the square), we use the formula:
\[
\text{Area of inscribed square} = \frac{1}{2} \times \text{side length of outer square}^2
\]
Substituting the given value:
\[
\text{Area} = \frac{1}{2} \times 20^2 = \frac{1}{2} \times 400 = 200 \, \text{m}^2
\]
Final Answer: The correct answer is (c) \( 200 \, \text{m}^2 \).