\(\sqrt2 :1\)

Let the length of the rectangle be \( l \) and breadth be \( b \).
The radius, \( \frac{l}{2} \), and \( b \) in the above diagram form a right-angled triangle.
Using the Pythagorean theorem:
\[ \left(\frac{l}{2}\right)^2 + b^2 = 2^2 \]
Area of the rectangle = \( l \times b \)
The area can be maximized by using the AM-GM (Arithmetic Mean – Geometric Mean) inequality or by setting the two squares equal to make their geometric mean maximum:
\[ \left(\frac{l}{2}\right)^2 = b^2 \]
Solving this gives:
\[ \frac{l}{2} = b \Rightarrow l = 2b \]
So, the ratio of length to breadth is:
\[ \frac{l}{b} = \frac{2}{1} \]
Therefore, the correct option is (D): \( 2:1 \)

For any natural number $k$, let $a_k = 3^k$. The smallest natural number $m$ for which \[ (a_1)^1 \times (a_2)^2 \times \dots \times (a_{20})^{20} \;<\; a_{21} \times a_{22} \times \dots \times a_{20+m} \] is: