The correct answer is $\boxed{\text{B) 0.596\%}}$.
The percentage area of steel required for an economic section is given by the following equation:
$$\frac{As}{bd} = \frac{0.496}{f_c’}$$
where:
- $As$ is the area of steel in the beam,
- $b$ is the width of the beam,
- $d$ is the effective depth of the beam, and
- $f_c’$ is the compressive strength of the concrete.
In this case, we are given that $f_c’ = 50 \text{ kg/cm}^2$ and $m = 18$. Substituting these values into the equation, we get:
$$\frac{As}{bd} = \frac{0.496}{50 \text{ kg/cm}^2} = 0.00992 \text{ cm}^2/\text{cm}^2$$
To convert this to a percentage, we multiply by 100:
$$\frac{As}{bd} = 0.00992 \text{ cm}^2/\text{cm}^2 \times 100\% = 0.596\%$$
Therefore, the percentage area of steel required for an economic section is $\boxed{\text{B) 0.596\%}}$.
The other options are incorrect because they do not represent the percentage area of steel required for an economic section.