Since rays are converging, so it is a converging lens i.e., convex lens. Rays coming from sun are considered to be coming from infinity and a converging lens converges all the incident rays from infinity at focus.
Convex lens forms real image of a distant object and its focal length is positive in sign.
So, here focal length $=20\mathrm{cm}=0.2\mathrm{m}$
Power of convex lens gives the degree of converging ability of the lens.
Power of lens is the reciprocal of focal length. Power $=\frac{1}{\mathit{f}}=\frac{100}{20}=5\mathrm{D}$
Therefore, power of convex lens with $20\mathrm{cm}$ focal length is $+5\mathrm{D}$.