1. Use Torricelli’s theorem

For a small hole at depth $h$ below the free surface of a liquid, the speed of efflux is

$v = \sqrt{2gh}$

2. Identify the depth

The cylinder is completely filled with water and the hole is near the bottom.

So the depth of the hole below the top free surface is approximately the full height:

$h = 20\,\text{m}$

3. Substitute values

Taking $g \approx 10\,\text{m s}^{-2}$,

$v = \sqrt{2 \times 10 \times 20}$

$v = \sqrt{400}$

$v = 20\,\text{m s}^{-1}$

4. Match with options

The correct option is:

$\boxed{20}$

So, Option B is correct.