B

To determine the focal length of a plano-convex lens, we can use the lens maker's formula:
$$ f = \frac{R}{(n - 1)} $$
where:
- $f$ is the focal length,
- $R$ is the radius of curvature of the lens,
- $n$ is the refractive index of the lens material.
Given:
- Radius of curvature, $R = 30 cm$
- Refractive index, $n = 1.5$.
Now substitute the values into the formula:
$$ f = \frac{30 cm}{(1.5 - 1)} $$
$$ f = \frac{30 cm}{0.5} $$
$$ f = 60 cm $$
Therefore, the focal length of the lens is 60 cm.