A lens of focal power 0.5 D is:

Power of lens = 0.5 D
The power of a lens is given by : $\mathrm{P} = \frac{1}{\mathrm{f}}$
where f is the focal length of the lens.

Substituting the given value in the following equation,

$\mathrm{P}=\frac{1}{\mathrm{f}}\Rightarrow \mathrm{f}=\frac{1}{0.5}=2\mathrm{m}$

The positive sign of the power and the focal length implies that it is a convex lens.

Therefore, it must be a convex lens with a focal length of 2 m.