1. Use power addition for lenses in contact

For thin lenses kept in contact, their powers add:

$P_{\text{total}} = P_1 + P_2 + P_3 + P_4 + P_5$

Since all 5 lenses are identical, let the power of each lens be $P$. Then,

$5P = 25\,\text{D}$

2. Find power of each lens

$P = \frac{25}{5} = 5\,\text{D}$

3. Use relation between power and focal length

$P = \frac{1}{f}$

where $f$ is in metres. So,

$f = \frac{1}{P} = \frac{1}{5}\,\text{m} = 0.2\,\text{m}$

4. Convert into cm

$0.2\,\text{m} = 20\,\text{cm}$

5. Match with options

The focal length of each convex lens is:

$\boxed{20\,\text{cm}}$

So the correct option is B.