A

Step 1: Use the lens maker's formula for a plano-convex lens, which is given by:
$$ f = \frac{R}{2(n - 1)} $$
where:
- $f$ is the focal length of the lens
- $R$ is the radius of curvature of the curved surface
- $n$ is the refractive index of the lens material
Step 2: Substitute the given values into the formula:
Radius of curvature, $R = 30 \text{ cm}$
Refractive index, $n = 1.5$
$$ f = \frac{30}{2(1.5 - 1)} $$
Step 3: Calculate the values inside the brackets:
$$ n - 1 = 1.5 - 1 = 0.5 $$
Step 4: Now substitute this value:
$$ f = \frac{30}{2 \times 0.5} = \frac{30}{1} = 30 \text{ cm} $$
Therefore, the focal length of the lens is 30 cm.